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

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Message-ID: <73c871d3d674607fafc7b79e602ec587@codeaurora.org>
Date:   Mon, 01 Feb 2021 21:20:07 +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 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.

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







> Thanks