linux-kernel - Re: [PATCH v7 1/3] dmaengine: Add support for APM X-Gene SoC DMA engine driver

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Date:	Mon, 16 Mar 2015 17:24:34 +0530
From:	Rameshwar Sahu <rsahu@....com>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	dan.j.williams@...el.com, dmaengine@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	jcm@...hat.com, patches@....com, Loc Ho <lho@....com>
Subject: Re: [PATCH v7 1/3] dmaengine: Add support for APM X-Gene SoC DMA
 engine driver

Hi Vinod,


On Mon, Mar 16, 2015 at 4:57 PM, Vinod Koul <vinod.koul@...el.com> wrote:
> On Mon, Mar 16, 2015 at 04:00:22PM +0530, Rameshwar Sahu wrote:
>
>> >> +static struct xgene_dma_desc_sw *xgene_dma_alloc_descriptor(
>> >> +                              struct xgene_dma_chan *chan)
>> >> +{
>> >> +     struct xgene_dma_desc_sw *desc;
>> >> +     dma_addr_t phys;
>> >> +
>> >> +     desc = dma_pool_alloc(chan->desc_pool, GFP_NOWAIT, &phys);
>> >> +     if (!desc) {
>> >> +             chan_dbg(chan, "Failed to allocate LDs\n");
>> > not error?
>>
>> Yes it's error only by lacking of dma memory, do I need to use dev_err
>> to show the error msg ??
> yes

Okay fine.
>
>>
>> >
>> >> +static void xgene_dma_free_desc_list_reverse(struct xgene_dma_chan *chan,
>> >> +                                          struct list_head *list)
>> > do we really care about free order?
>>
>> Yes it start dellocation of descriptor by tail.
> and why by tail is not clear.

We can free allocated descriptor in forward order from head or in
reverse order, I just followed here fsldma.c driver.
Does this make sense ??


>
>> > where are you mapping dma buffers?
>>
>>  I didn't get you here. Can you please explain me here what you mean.
>> As per my understanding client should map the dma buffer and give the
>> physical address and size to this callback prep routines.
> not for memcpy, that is true for slave transfers
>
> For mempcy the idea is that drivers will do buffer mapping

Still I am clear here, why memcpy will do buffer mapping, I see other
drivers and also async_memcpy.c , they only map it and pass mapped
physical dma address to driver.

Buffer mapping mean you here is dma_map_xxx ?? Am I correct.

>
>> > why are you calling this here, status check shouldnt do this...
>>
>> Okay, I will remove it.
>>
>>
>> >> +                     spin_unlock_bh(&chan->lock);
>> >> +                     return DMA_IN_PROGRESS;
>> > residue here is size of transacation.
>>
>> We can't calculate here residue size. We don't have any controller
>> register which will tell about remaining transaction size.
> Okay if you cant calculate residue why do we have this fn?

So basically case here for me is completion of dma descriptor
submitted to hw is not same as order of submission to hw.
So scenario coming in multithread running :e.g. let's assume we have
submitted two descriptors first has cookie 1001 and second has 1002,
now 1002 is completed first, so updated last_completed_cookie as 1002
but not yer checked for dma_tx_status, and then first cookie completes
and update last_completed_cookie as 1001, now second transaction check
for tx_status and it get DMA_IN_PROGRESS, because
last_completed_cookie(1001) is less than second transaction's
cookie(1002).

Due to this issue I am traversing that transaction in pending list and
running list, if not there means we are done.

Does this make sense??

>
>>
>> >> +             }
>> >> +     }
>> >> +
>> >> +     /* Check if this descriptor is in running queue */
>> >> +     list_for_each_entry(desc, &chan->ld_running, node) {
>> >> +             if (desc->tx.cookie == cookie) {
>> >> +                     /* Cleanup any running and executed descriptors */
>> >> +                     xgene_dma_cleanup_descriptors(chan);
>> > ditto?
>>
>> Okay
>>
>>
>> >> +                     spin_unlock_bh(&chan->lock);
>> >> +                     return dma_cookie_status(&chan->dma_chan,
>> >> +                                              cookie, txstate);
>> > and you havent touched txstate so what is the intent here...?
>>
>> txstate can filled by caller, so it may be NULL or not NULL, we are
>> passing same.
> something seems very wrong here. Status should return the current satue of
> queried descriptor and if residue value in txstate, you seem to be doing
> something else, quotesion is what and why :)
>

Please see my above comment.
Thanks
> --
> ~Vinod
>
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