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Date:   Sun, 13 Dec 2020 10:54:26 +0100
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Sowjanya Komatineni <skomatineni@...dia.com>
Cc:     <thierry.reding@...il.com>, <jonathanh@...dia.com>,
        <broonie@...nel.org>, <robh+dt@...nel.org>, <lukas@...ner.de>,
        <bbrezillon@...nel.org>, <p.yadav@...com>,
        <tudor.ambarus@...rochip.com>, <linux-spi@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH v3 5/9] spi: spi-mem: Allow masters to transfer dummy
 cycles directly by hardware

On Sat, 12 Dec 2020 09:28:50 -0800
Sowjanya Komatineni <skomatineni@...dia.com> wrote:

> On 12/12/20 2:57 AM, Boris Brezillon wrote:
> > On Fri, 11 Dec 2020 13:15:59 -0800
> > Sowjanya Komatineni <skomatineni@...dia.com> wrote:
> >  
> >> This patch adds a flag SPI_MASTER_USES_HW_DUMMY_CYCLES for the controllers
> >> that support transfer of dummy cycles by the hardware directly.  
> > Hm, not sure this is a good idea. I mean, if we expect regular SPI
> > devices to use this feature, then why not, but if it's just for
> > spi-mem, I'd recommend implementing a driver-specific exec_op() instead
> > of using the default one.  
> 
> dummy cycles programming is SPI device specific.
> 
> Transfer of dummy bytes by SW or HW controller can be depending on 
> features supported by controller.
> 
> Adding controller driver specific exec_op() Just for skipping dummy 
> bytes transfer will have so much of redundant code pretty much what all 
> spi_mem_exec_op does.
> 
> So in v1, I handled this in controller driver by skipping SW transfer of 
> dummy bytes during dummy phase and programming dummy cycles in 
> controller register to allow HW to transfer.
> 
> Based on v1 feedback discussion, added this flag 
> SPI_MASTER_USES_HW_DUMMY_CYCLES which can be used by controllers 
> supporting HW dummy bytes transfer and updated spi_mem_exec_op to skip 
> SW dummy bytes.
> 
> This helps other controllers supporting HW transfer of dummy bytes as 
> well just to set the flag and use dummy cycles directly.

Except saying a spi_message has X dummy cycle is not precise enough.
Where are those dummy cycles in the transfer sequence? spi-mem has well
defined sequencing (cmd[+addr][+dummy][+data]) so we know exacly where
dummy cycles are, but trying to retro-fit the dummy-cycle concept in
the generic spi_message is confusing IMHO. If want to avoid code
duplication, I'm pretty sure the driver can be reworked so the
spi_transfer/exec_op() path can share most of the logic (that probably
implies declaring a tegra_qspi_op).

> 
> > If we go for those core changes, we should at least add a
> > ctrl->max_dummy_cycles field so the core can fallback to regular writes
> > when the number of dummy cycles in the spi_mem_op exceeds what the
> > controller can do.  
> Yes makes sense. Will add this once we decide on keeping this flag to 
> identify controllers supporting HW transfer of dummy bytes Vs SW transfer.
> >> For controller with this flag set, spi-mem driver will skip dummy bytes
> >> transfer in the spi message.
> >>
> >> Controller drivers can get the number of dummy cycles from spi_message.
> >>
> >> Signed-off-by: Sowjanya Komatineni <skomatineni@...dia.com>
> >> ---
> >>   drivers/spi/spi-mem.c   | 18 +++++++++++-------
> >>   include/linux/spi/spi.h |  8 ++++++++
> >>   2 files changed, 19 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
> >> index f3a3f19..38a523b 100644
> >> --- a/drivers/spi/spi-mem.c
> >> +++ b/drivers/spi/spi-mem.c
> >> @@ -350,13 +350,17 @@ int spi_mem_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
> >>   	}
> >>   
> >>   	if (op->dummy.nbytes) {
> >> -		memset(tmpbuf + op->addr.nbytes + 1, 0xff, op->dummy.nbytes);
> >> -		xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
> >> -		xfers[xferpos].len = op->dummy.nbytes;
> >> -		xfers[xferpos].tx_nbits = op->dummy.buswidth;
> >> -		spi_message_add_tail(&xfers[xferpos], &msg);
> >> -		xferpos++;
> >> -		totalxferlen += op->dummy.nbytes;
> >> +		if (ctlr->flags & SPI_MASTER_USES_HW_DUMMY_CYCLES) {
> >> +			msg.dummy_cycles = (op->dummy.nbytes * 8) / op->dummy.buswidth;
> >> +		} else {
> >> +			memset(tmpbuf + op->addr.nbytes + 1, 0xff, op->dummy.nbytes);
> >> +			xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
> >> +			xfers[xferpos].len = op->dummy.nbytes;
> >> +			xfers[xferpos].tx_nbits = op->dummy.buswidth;
> >> +			spi_message_add_tail(&xfers[xferpos], &msg);
> >> +			xferpos++;
> >> +			totalxferlen += op->dummy.nbytes;
> >> +		}
> >>   	}
> >>   
> >>   	if (op->data.nbytes) {
> >> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> >> index aa09fdc..2024149 100644
> >> --- a/include/linux/spi/spi.h
> >> +++ b/include/linux/spi/spi.h
> >> @@ -512,6 +512,8 @@ struct spi_controller {
> >>   
> >>   #define SPI_MASTER_GPIO_SS		BIT(5)	/* GPIO CS must select slave */
> >>   
> >> +#define SPI_MASTER_USES_HW_DUMMY_CYCLES	BIT(6)	/* HW dummy bytes transfer */
> >> +
> >>   	/* flag indicating this is an SPI slave controller */
> >>   	bool			slave;
> >>   
> >> @@ -1022,6 +1024,12 @@ struct spi_message {
> >>   	unsigned		actual_length;
> >>   	int			status;
> >>   
> >> +	/*
> >> +	 * dummy cycles in the message transfer. This is used by the controller
> >> +	 * drivers supports transfer of dummy cycles directly by the hardware.
> >> +	 */
> >> +	u8			dummy_cycles;
> >> +
> >>   	/* for optional use by whatever driver currently owns the
> >>   	 * spi_message ...  between calls to spi_async and then later
> >>   	 * complete(), that's the spi_controller controller driver.  

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