linux-kernel - Re: [PATCH 2/5] dmaengine: Add support for custom data mapping

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Message-ID: <5332b039ab52d2ef7353121615871757@codeaurora.org>
Date:   Thu, 19 Jan 2017 20:27:43 +0530
From:   Abhishek Sahu <absahu@...eaurora.org>
To:     Andy Gross <andy.gross@...aro.org>
Cc:     Vinod Koul <vinod.koul@...el.com>, dan.j.williams@...el.com,
        stanimir.varbanov@...aro.org, mcgrof@...e.com,
        okaya@...eaurora.org, pramod.gurav@...aro.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH 2/5] dmaengine: Add support for custom data mapping

On 2017-01-19 19:43, Andy Gross wrote:
> On Thu, Jan 19, 2017 at 10:31:50AM +0530, Vinod Koul wrote:
> 
> <snip>
> 
>> > > >
>> > > >I really think that we need some additional API that allows for the flag
>> > > >munging
>> > > >for the descriptors instead of overriding the prep_slave_sg.  We already
>> > > >needed
>> > > >to change the way the flags are passed anyway.  And instead of building up
>> > > >a
>> > > >special sg list, the API should take a structure that has a 1:1 mapping of
>> > > >the
>> > > >flags to the descriptors.  And you would call this API on your descriptor
>> > > >before
>> > > >issuing it.
>> 
>> Munging wont be a good idea, but for some of the cases current flags 
>> can be
>> used, and if need be, we can add additional flags
> 
> Is adding flags a possibility?  I tried to match up BAM flags to ones 
> that made
> sense that were currently defined, but adding a CMD flag would be kind 
> of odd.
> 
> It was kind of a stretch to use the PREP_FENCE for the notify when done 
> flag.
> 
>> > > >
>> > > >So build up the sglist.  Call the prep_slave_sg.  You get back a tx
>> > > >descriptor
>> > > >that underneath is a bam descriptor.  Then call the API giving the
>> > > >descriptor
>> > > >and the structure that defines the flags for the descriptors.  Then submit
>> > > >the
>> > > >descriptor.
>> > > >
>> > > >Something like:
>> > > >int qcom_bam_apply_descriptor_flags(struct dma_async_tx_descriptor *tx,
>> > > >				    u16 *flags)
>> > > >{
>> > > >	struct bam_async_desc async_desc = container_of(tx,
>> > > >							struct bam_async_desc,
>> > > >							vd.tx);
>> > > >	int i;
>> > > >
>> > > >	for (i = 0; i < async_desc->num_desc; i++)
>> > > >		async_desc->desc[i].flags = cpu_to_le16(flags[i]);
>> > > >}
>> > > >
>> > > >EXPORT_SYMBOL(qcom_bam_apply_descriptor_flags)
>> 
>> This makes it bam specific and causes issues if we want to use this 
>> code in
>> generic libs, but yes this is an option but should be last resort.
> 
> If adding flags is a possibility (which it didn't seem to be in the 
> past), that
> would make things much easier.
Also, Main reason for this approach was to set different flags for each
BAM descriptor present in a TX descriptor.

> 
> Regards,
> 
> Andy

-- 
Abhishek Sahu