Message-ID: <iang2jpe4s6wmbypmtq5uswcm6n6xntqdulyhekcz5k6zxddu3@re3rrr4dso5p>
Date: Tue, 22 Jul 2025 15:46:12 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Jyothi Kumar Seerapu <quic_jseerapu@...cinc.com>
Cc: Vinod Koul <vkoul@...nel.org>,
        Mukesh Kumar Savaliya <quic_msavaliy@...cinc.com>,
        Viken Dadhaniya <quic_vdadhani@...cinc.com>,
        Andi Shyti <andi.shyti@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        linux-arm-msm@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, quic_vtanuku@...cinc.com
Subject: Re: [PATCH v6 2/2] i2c: i2c-qcom-geni: Add Block event interrupt
 support

On Tue, Jul 22, 2025 at 05:50:08PM +0530, Jyothi Kumar Seerapu wrote:
> 
> 
> On 7/19/2025 3:27 PM, Dmitry Baryshkov wrote:
> > On Mon, Jul 07, 2025 at 09:58:30PM +0530, Jyothi Kumar Seerapu wrote:
> > > 
> > > 
> > > On 7/4/2025 1:11 AM, Dmitry Baryshkov wrote:
> > > > On Thu, 3 Jul 2025 at 15:51, Jyothi Kumar Seerapu
> > > > <quic_jseerapu@...cinc.com> wrote:
> > > > > 
> > > > > 
> > > > > 
> > > > > On 6/19/2025 9:46 PM, Jyothi Kumar Seerapu wrote:
> > > > > > 
> > > > > > 
> > > > > > On 6/18/2025 1:02 AM, Dmitry Baryshkov wrote:
> > > > > > > On Tue, 17 Jun 2025 at 17:11, Jyothi Kumar Seerapu
> > > > > > > <quic_jseerapu@...cinc.com> wrote:
> > > > > > > > 
> > > > > > > > 
> > > > > > > > 
> > > > > > > > On 5/30/2025 10:12 PM, Dmitry Baryshkov wrote:
> > > > > > > > > On Fri, May 30, 2025 at 07:36:05PM +0530, Jyothi Kumar Seerapu wrote:
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > On 5/21/2025 6:15 PM, Dmitry Baryshkov wrote:
> > > > > > > > > > > On Wed, May 21, 2025 at 03:58:48PM +0530, Jyothi Kumar Seerapu wrote:
> > > > > > > > > > > > 
> > > > > > > > > > > > 
> > > > > > > > > > > > On 5/9/2025 9:31 PM, Dmitry Baryshkov wrote:
> > > > > > > > > > > > > On 09/05/2025 09:18, Jyothi Kumar Seerapu wrote:
> > > > > > > > > > > > > > Hi Dimitry, Thanks for providing the review comments.
> > > > > > > > > > > > > > 
> > > > > > > > > > > > > > On 5/6/2025 5:16 PM, Dmitry Baryshkov wrote:
> > > > > > > > > > > > > > > On Tue, May 06, 2025 at 04:48:44PM +0530, Jyothi Kumar Seerapu
> > > > > > > > > > > > > > > wrote:
> > > > > > > > > > > > > > > > The I2C driver gets an interrupt upon transfer completion.
> > > > > > > > > > > > > > > > When handling multiple messages in a single transfer, this
> > > > > > > > > > > > > > > > results in N interrupts for N messages, leading to significant
> > > > > > > > > > > > > > > > software interrupt latency.
> > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > To mitigate this latency, utilize Block Event Interrupt (BEI)
> > > > > > > > > > > > > > > > mechanism. Enabling BEI instructs the hardware to prevent
> > > > > > > > > > > > > > > > interrupt
> > > > > > > > > > > > > > > > generation and BEI is disabled when an interrupt is necessary.
> > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > Large I2C transfer can be divided into chunks of 8 messages
> > > > > > > > > > > > > > > > internally.
> > > > > > > > > > > > > > > > Interrupts are not expected for the first 7 message
> > > > > > > > > > > > > > > > completions, only
> > > > > > > > > > > > > > > > the last message triggers an interrupt, indicating the
> > > > > > > > > > > > > > > > completion of
> > > > > > > > > > > > > > > > 8 messages. This BEI mechanism enhances overall transfer
> > > > > > > > > > > > > > > > efficiency.
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > Why do you need this complexity? Is it possible to set the
> > > > > > > > > > > > > > > DMA_PREP_INTERRUPT flag on the last message in the transfer?
> > > > > > > > > > > > > > 
> > > > > > > > > > > > > > If i undertsand correctly, the suggestion is to get the single
> > > > > > > > > > > > > > intetrrupt for last i2c message only.
> > > > > > > > > > > > > > 
> > > > > > > > > > > > > > But With this approach, we can't handle large number of i2c
> > > > > > > > > > > > > > messages
> > > > > > > > > > > > > > in the transfer.
> > > > > > > > > > > > > > 
> > > > > > > > > > > > > > In GPI driver, number of max TREs support is harcoded to 64
> > > > > > > > > > > > > > (#define
> > > > > > > > > > > > > > CHAN_TRES   64) and for I2C message, we need Config TRE, GO TRE
> > > > > > > > > > > > > > and
> > > > > > > > > > > > > > DMA TREs. So, the avilable TREs are not sufficient to handle
> > > > > > > > > > > > > > all the
> > > > > > > > > > > > > > N messages.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > It sounds like a DMA driver issue. In other words, the DMA
> > > > > > > > > > > > > driver can
> > > > > > > > > > > > > know that it must issue an interrupt before exausting 64 TREs in
> > > > > > > > > > > > > order
> > > > > > > > > > > > > to
> > > > > > > > > > > > > 
> > > > > > > > > > > > > > 
> > > > > > > > > > > > > > Here, the plan is to queue i2c messages (QCOM_I2C_GPI_MAX_NUM_MSGS
> > > > > > > > > > > > > > or 'num' incase for less messsages), process and unmap/free
> > > > > > > > > > > > > > upon the
> > > > > > > > > > > > > > interrupt based on QCOM_I2C_GPI_NUM_MSGS_PER_IRQ.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > Why? This is some random value which has no connection with
> > > > > > > > > > > > > CHAN_TREs.
> > > > > > > > > > > > > Also, what if one of the platforms get a 'liter' GPI which
> > > > > > > > > > > > > supports less
> > > > > > > > > > > > > TREs in a single run? Or a super-premium platform which can use 256
> > > > > > > > > > > > > TREs? Please don't workaround issues from one driver in another
> > > > > > > > > > > > > one.
> > > > > > > > > > > > 
> > > > > > > > > > > > We are trying to utilize the existing CHAN_TRES mentioned in the
> > > > > > > > > > > > GPI driver.
> > > > > > > > > > > > With the following approach, the GPI hardware can process N
> > > > > > > > > > > > number of I2C
> > > > > > > > > > > > messages, thereby improving throughput and transfer efficiency.
> > > > > > > > > > > > 
> > > > > > > > > > > > The main design consideration for using the block event interrupt
> > > > > > > > > > > > is as
> > > > > > > > > > > > follows:
> > > > > > > > > > > > 
> > > > > > > > > > > > Allow the hardware to process the TREs (I2C messages), while the
> > > > > > > > > > > > software
> > > > > > > > > > > > concurrently prepares the next set of TREs to be submitted to the
> > > > > > > > > > > > hardware.
> > > > > > > > > > > > Once the TREs are processed, they can be freed, enabling the
> > > > > > > > > > > > software to
> > > > > > > > > > > > queue new TREs. This approach enhances overall optimization.
> > > > > > > > > > > > 
> > > > > > > > > > > > Please let me know if you have any questions, concerns, or
> > > > > > > > > > > > suggestions.
> > > > > > > > > > > 
> > > > > > > > > > > The question was why do you limit that to
> > > > > > > > > > > QCOM_I2C_GPI_NUM_MSGS_PER_IRQ.
> > > > > > > > > > > What is the reason for that limit, etc. If you think about it, The
> > > > > > > > > > > GENI
> > > > > > > > > > > / I2C doesn't impose any limit on the number of messages processed in
> > > > > > > > > > > one go (if I understand it correctly). Instead the limit comes
> > > > > > > > > > > from the
> > > > > > > > > > > GPI DMA driver. As such, please don't add extra 'handling' to the I2C
> > > > > > > > > > > driver. Make GPI DMA driver responsible for saying 'no more for now',
> > > > > > > > > > > then I2C driver can setup add an interrupt flag and proceed with
> > > > > > > > > > > submitting next messages, etc.
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > For I2C messages, we need to prepare TREs for Config, Go and DMAs.
> > > > > > > > > > However,
> > > > > > > > > > if a large number of I2C messages are submitted then may may run
> > > > > > > > > > out of
> > > > > > > > > > memory for serving the TREs. The GPI channel supports a maximum of
> > > > > > > > > > 64 TREs,
> > > > > > > > > > which is insufficient to serve 32 or even 16 I2C messages
> > > > > > > > > > concurrently,
> > > > > > > > > > given the multiple TREs required per message.
> > > > > > > > > > 
> > > > > > > > > > To address this limitation, a strategy has been implemented to
> > > > > > > > > > manage how
> > > > > > > > > > many messages can be queued and how memory is recycled. The constant
> > > > > > > > > > QCOM_I2C_GPI_MAX_NUM_MSGS is set to 16, defining the upper limit of
> > > > > > > > > > messages that can be queued at once. Additionally,
> > > > > > > > > > QCOM_I2C_GPI_NUM_MSGS_PER_IRQ is set to 8, meaning that
> > > > > > > > > > half of the queued messages are expected to be freed or deallocated
> > > > > > > > > > per
> > > > > > > > > > interrupt.
> > > > > > > > > > This approach ensures that the driver can efficiently manage TRE
> > > > > > > > > > resources
> > > > > > > > > > and continue queuing new I2C messages without exhausting memory.
> > > > > > > > > > > I really don't see a reason for additional complicated handling in
> > > > > > > > > > > the
> > > > > > > > > > > geni driver that you've implemented. Maybe I misunderstand
> > > > > > > > > > > something. In
> > > > > > > > > > > such a case it usually means that you have to explain the design
> > > > > > > > > > > in the
> > > > > > > > > > > commit message / in-code comments.
> > > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > The I2C Geni driver is designed to prepare and submit descriptors
> > > > > > > > > > to the GPI
> > > > > > > > > > driver one message at a time.
> > > > > > > > > > As a result, the GPI driver does not have visibility into the current
> > > > > > > > > > message index or the total number of I2C messages in a transfer.
> > > > > > > > > > This lack
> > > > > > > > > > of context makes it challenging to determine when to set the block
> > > > > > > > > > event
> > > > > > > > > > interrupt, which is typically used to signal the completion of a
> > > > > > > > > > batch of
> > > > > > > > > > messages.
> > > > > > > > > > 
> > > > > > > > > > So, the responsibility for deciding when to set the BEI should lie
> > > > > > > > > > with the
> > > > > > > > > > I2C driver.
> > > > > > > > > > 
> > > > > > > > > > If this approach is acceptable, I will proceed with updating the
> > > > > > > > > > relevant
> > > > > > > > > > details in the commit message.
> > > > > > > > > > 
> > > > > > > > > > Please let me know if you have any concerns or suggestions.
> > > > > > > > > 
> > > > > > > > Hi Dmitry, Sorry for the delayed response, and thank you for the
> > > > > > > > suggestions.
> > > > > > > > 
> > > > > > > > > - Make gpi_prep_slave_sg() return NULL if flags don't have
> > > > > > > > >       DMA_PREP_INTERRUPT flag and there are no 3 empty TREs for the
> > > > > > > > >       interrupt-enabled transfer.
> > > > > > > > "there are no 3 empty TREs for the interrupt-enabled transfer."
> > > > > > > > Could you please help me understand this a bit better?
> > > > > > > 
> > > > > > > In the GPI driver you know how many TREs are available. In
> > > > > > > gpi_prep_slave_sg() you can check that and return an error if there
> > > > > > > are not enough TREs available.
> > > > > > > 
> > > > > > > > > 
> > > > > > > > > - If I2C driver gets NULL from dmaengine_prep_slave_single(), retry
> > > > > > > > >       again, adding DMA_PREP_INTERRUPT. Make sure that the last one
> > > > > > > > > always
> > > > > > > > >       gets DMA_PREP_INTERRUPT.
> > > > > > > > Does this mean we need to proceed to the next I2C message and ensure
> > > > > > > > that the DMA_PREP_INTERRUPT flag is set for the last I2C message in each
> > > > > > > > chunk? And then, should we submit the chunk of messages to the GSI
> > > > > > > > hardware for processing?
> > > > > > > 
> > > > > > > No. You don't have to peek at the next I2C message. This all concerns
> > > > > > > the current I2C message. The only point where you have to worry is to
> > > > > > > explicitly set the flag for the last message.
> > > > > > > 
> > > > > > > > 
> > > > > > > > > 
> > > > > > > > > - In geni_i2c_gpi_xfer() split the loop to submit messages until you
> > > > > > > > >       can, then call wait_for_completion_timeout() and then
> > > > > > > > >       geni_i2c_gpi_unmap() for submitted messages, then continue with
> > > > > > > > > a new
> > > > > > > > >       portion of messages.
> > > > > > > > Since the GPI channel supports a maximum of 64 TREs, should we consider
> > > > > > > > submitting a smaller number of predefined messages — perhaps fewer than
> > > > > > > > 32, such as 16?
> > > > > > > 
> > > > > > > Why? Just submit messages until they fit, then flush the DMA async
> > > > > > > channel.
> > > > > > > 
> > > > > > > > This is because handling 32 messages would require one TRE for config
> > > > > > > > and 64 TREs for the Go and DMA preparation steps, which exceeds the
> > > > > > > > channel's TRE capacity of 64.
> > > > > > > > 
> > > > > > > > We designed the approach to submit a portion of the messages — for
> > > > > > > > example, 16 at a time. Once 8 messages are processed and freed, the
> > > > > > > > hardware can continue processing the TREs, while the software
> > > > > > > > simultaneously prepares the next set of TREs. This parallelism helps in
> > > > > > > > efficiently utilizing the hardware and enhances overall system
> > > > > > > > optimization.
> > > > > > > 
> > > > > > > 
> > > > > > > And this overcomplicates the driver and introduces artificial
> > > > > > > limitations which need explanation. Please fix it in a simple way
> > > > > > > first. Then you can e.g. implement the watermark at the half of the
> > > > > > > GPI channel depth and request DMA_PREP_INTERRUPT to be set in the
> > > > > > > middle of the full sequence, allowing it to be used asynchronously in
> > > > > > > the background.
> > > > > > > 
> > > > > > 
> > > > > > Okay, will review it. Thanks.
> > > > > > 
> > > > > > 
> > > > > 
> > > > > Hi Dmitry,
> > > > > 
> > > > > Can you please check and confirm the approach to follow is something
> > > > > like the pseudo code mentioned below:
> > > > 
> > > > Yes, this is what I've had in mind.
> > > 
> > > So, Apart from the changes related to "submitting I2C messages until they
> > > fit" and "unmapping all processed I2C messages together", the rest of the
> > > code looks remains the same as in the v6 patch ?
> > > Also, in the GPI driver, we need to add logic to retrieve the number of
> > > available TREs.
> > > 
> > > I have a concern regarding throughput and achieving parallelism between
> > > software and hardware processing with this new approach. Since we need to
> > > unmap all processed messages together, the software cannot queue the next
> > > set of TREs while the hardware is still processing the current ones.
> > 
> > Does that warrant the over-complexity of the driver or close-coupling of
> > I2C and GPE drivers?
> > 
> > The I2C is a slow bus and it is not expected to be used for
> > high-throughput data.
> > 
> The block event interrupt and multi-descriptor handling are primarily added
> to support a camera use case, where multiple registers are need to be
> configured in a single I2C transfer with 200 or more i2c messages. This
> enhancement is expected to improve throughput and meet performance KPIs.
> 
> > > 
> > > As I mentioned earlier, the previous approach allowed partial unmapping
> > > where half of the messages processed by the hardware could be
> > > freed/unmapped. This enabled the hardware to continue processing the
> > > remaining TREs while the software simultaneously prepared the next batch.
> > > This parallelism helped in better hardware utilization and improved overall
> > > system performance.
> > 
> > Measurements / values / impact?
> > 
> > > 
> > > Could you please confirm if can go with the similar approach of unmap the
> > > processed TREs based on a fixed threshold or constant value, instead of
> > > unmapping them all at once?
> > 
> > I'd still say, that's a bad idea. Please stay within the boundaries of
> > the DMA API.
> >
> I agree with the approach you suggested—it's the GPI's responsibility to
> manage the available TREs.
> 
> However, I'm curious whether can we set a dynamic watermark value perhaps
> half the available TREs) to trigger unmapping of processed TREs ? This would
> allow the software to prepare the next set of TREs while the hardware
> continues processing the remaining ones, enabling better parallelism and
> throughput.

Let's land the simple implementation first, which can then be improved.
However I don't see any way to return 'above the watermark' from the DMA
controller. You might need to enhance the API.

> 
> > > > 
> > > > > 
> > > > > GPI driver:
> > > > > In gpi_prep_slave_sg() function,
> > > > > 
> > > > > if (!(flags & DMA_PREP_INTERRUPT) && !gpi_available_tres(chan))
> > > > >           return NULL;
> > > > > 
> > > > > 
> > > > > I2C GENI driver:
> > > > > 
> > > > > for (i = 0; i < num; i++)
> > > > > {
> > > > >       /* Always set interrupt for the last message */
> > > > >       if (i == num_msgs - 1)
> > > > >           flags |= DMA_PREP_INTERRUPT;
> > > > > 
> > > > > 
> > > > >       desc = dmaengine_prep_slave_single(chan, dma_addr, len, dir, flags);
> > > > >       if (!desc && !(flags & DMA_PREP_INTERRUPT)) {
> > > > >             /* Retry with interrupt if not enough TREs */
> > > > >             flags |= DMA_PREP_INTERRUPT;
> > > > >             desc = dmaengine_prep_slave_single(chan, dma_addr, len, dir,   flags);
> > > > >       }
> > > > > 
> > > > > 
> > > > >       if (!desc)
> > > > >           break;
> > > > > 
> > > > > 
> > > > >        dmaengine_submit(desc);
> > > > >        msg_idx++;
> > > > > }
> > > > > 
> > > > > dma_async_issue_pending(chan));
> > > > > 
> > > > > time_left = wait_for_completion_timeout(&gi2c->done, XFER_TIMEOUT);
> > > > > if (!time_left)
> > > > >           return -ETIMEDOUT;
> > > > > 
> > > > > Now Invoke "geni_i2c_gpi_unmap" for unmapping all submitted I2C messages.
> > > > > 
> > > > > 
> > > > > Thanks,
> > > > > JyothiKumar
> > > > > 
> > > > > 
> > > > > 
> > > > 
> > > > 
> > > 
> > 
> 

-- 
With best wishes
Dmitry

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