linux-kernel - Re: [PATCH] dmaengine: qcom: bam_dma: Add LOCK and UNLOCK flag bit support

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Message-ID: <755d79fe46d819a4c5aaab245ab00634@codeaurora.org>
Date:   Tue, 09 Feb 2021 22:09:20 +0530
From:   mdalam@...eaurora.org
To:     Vinod Koul <vkoul@...nel.org>
Cc:     corbet@....net, agross@...nel.org, bjorn.andersson@...aro.org,
        dan.j.williams@...el.com, dmaengine@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, sricharan@...eaurora.org,
        mdalam=codeaurora.org@...eaurora.org
Subject: Re: [PATCH] dmaengine: qcom: bam_dma: Add LOCK and UNLOCK flag bit
 support

On 2021-02-01 21:20, mdalam@...eaurora.org wrote:
> On 2021-02-01 12:13, Vinod Koul wrote:
>> On 01-02-21, 11:52, mdalam@...eaurora.org wrote:
>>> On 2021-02-01 11:35, Vinod Koul wrote:
>>> > On 27-01-21, 23:56, mdalam@...eaurora.org wrote:
>> 
>>> > >   The actual LOCK/UNLOCK flag should be set on hardware command
>>> > > descriptor.
>>> > >   so this flag setting should be done in DMA engine driver. The user
>>> > > of the
>>> > > DMA
>>> > >   driver like (in case of IPQ5018) Crypto can use flag
>>> > > "DMA_PREP_LOCK" &
>>> > > "DMA_PREP_UNLOCK"
>>> > >   while preparing CMD descriptor before submitting to the DMA
>>> > > engine. In DMA
>>> > > engine driver
>>> > >   we are checking these flasgs on CMD descriptor and setting actual
>>> > > LOCK/UNLOCK flag on hardware
>>> > >   descriptor.
>>> >
>>> >
>>> > I am not sure I comprehend this yet.. when is that we would need to do
>>> > this... is this for each txn submitted to dmaengine.. or something
>>> > else..
>>> 
>>>  Its not for each transaction submitted to dmaengine. We have to set 
>>> this
>>> only
>>>  once on CMD descriptor. So when A53 crypto driver need to change the 
>>> crypto
>>> configuration
>>>  then first it will lock the all other pipes using setting the LOCK 
>>> flag bit
>>> on CMD
>>>  descriptor and then it can start the transaction , on data 
>>> descriptor this
>>> flag will
>>>  not get set once all transaction will be completed the A53 crypto 
>>> driver
>>> release the lock on
>>>  all other pipes using UNLOCK flag on CMD descriptor. So LOCK/UNLOCK 
>>> will be
>>> only once and not for
>>>  the each transaction.
>> 
>> Okay so why cant the bam driver check cmd descriptor and do 
>> lock/unlock
>> as below, why do we need users to do this.
>> 
>>         if (flags & DMA_PREP_CMD) {
>>                 do_lock_bam();
> 
>  User will not decide to do this LOCK/UNLOCK mechanism. It depends on 
> use case.
>  This LOCK/UNLOCK mechanism not required always. It needs only when
> hardware will be shared
>  between different core with different driver.
>  The LOCK/UNLOCK flags provides SW to enter ordering between pipes 
> execution.
>  (Generally, the BAM pipes are total independent from each other and
> work in parallel manner).
>  This LOCK/UNLOCK flags are part of actual pipe hardware descriptor.
> 
>  Pipe descriptor having the following flags:
>  INT : Interrupt
>  EOT: End of transfer
>  EOB: End of block
>  NWD: Notify when done
>  CMD: Command
>  LOCK: Lock
>  UNLOCK: Unlock
>  etc.
> 
>  Here the BAM driver is common driver for (QPIC, Crypto, QUP etc. in 
> IPQ5018)
>  So here only Crypto will be shared b/w multiple cores so For crypto
> request only the LOCK/UNLOCK
>  mechanism required.
>  For other request like for QPIC driver, QUPT driver etc. its not
> required. So Crypto driver has to raise the flag for
>  LOCK/UNLOCK while preparing CMD descriptor. The actual locking will
> happen in BAM driver only using condition
>  if (flags & DMA_PREP_CMD) {
>      if (flags & DMA_PREP_LOCK)
>         desc->flags |= cpu_to_le16(DESC_FLAG_LOCK);
>  }
> 
>  So Crypto driver should set this flag DMA_PREP_LOCK while preparing
> CMD descriptor.
>  So LOCK should be set on actual hardware pipe descriptor with
> descriptor type CMD.
> 
>> 
>> The point here is that this seems to be internal to dma and should be
>> handled by dma driver.
>> 
>   This LOCK/UNLOK flags are part of actual hardware descriptor so this
> should be handled by BAM driver only.
>   If we set condition like this
>   if (flags & DMA_PREP_CMD) {
>                 do_lock_bam();
>   Then LOCK/UNLOCK will be applied for all the CMD descriptor
> including (QPIC driver, QUP driver , Crypto driver etc.).
>   So this is not our intension. So we need to set this LOCK/UNLOCK
> only for the drivers it needs. So Crypto driver needs
>   locking mechanism so we will set LOCK/UNLOCK flag on Crypto driver
> request only for other driver request like QPIC driver,
>   QUP driver will not set this.
> 

    ping! Do you need any further info on this?

>> Also if we do this, it needs to be done for specific platforms..
>> 
> 
> 
> 
> 
> 
> 
> 
>> Thanks