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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Wed, 4 Jul 2018 13:38:58 +0300
Message-ID: <CAHp75Vfy8=i3dSYoQ5Uto-RtmU6E8BVWU1dmG8BihN=Wtkr+_A@mail.gmail.com>
Subject: Re: [PATCH] dma: add support for reporting pause and resume separately
To:     Vinod <vkoul@kernel.org>
Cc:     Marek Szyprowski <m.szyprowski@samsung.com>,
        Mark Brown <broonie@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>,
        dmaengine <dmaengine@vger.kernel.org>,
        ALSA Development Mailing List <alsa-devel@alsa-project.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
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On Wed, Jul 4, 2018 at 10:00 AM, Vinod <vkoul@kernel.org> wrote:
> Hi Marek,
>
> On 02-07-18, 15:08, Marek Szyprowski wrote:
>> 'cmd_pause' DMA channel capability means that respective DMA engine
>> supports both pausing and resuming given DMA channel. However, in some
>> cases it is important to know if DMA channel can be paused without the
>> need to resume it. This is a typical requirement for proper residue
>> reading on transfer timeout in UART drivers. There are also some DMA
>> engines with limited hardware, which doesn't really support resuming.
>
> Am curious given that your hardware does not support resume, what was motivation
> for adding pause?
>
>> Reporting pause and resume capabilities separately allows UART drivers to
>> properly check for the really required capabilities and operate in DMA
>> mode also in systems with limited DMA hardware. On the other hand drivers,
>> which rely on full channel suspend/resume support, should now check for
>> both 'pause' and 'resume' features.
>>
>> Existing clients of dma_get_slave_caps() have been checked and the only
>> driver which rely on proper channel resuming is soc-generic-dmaengine-pcm
>> driver, which has been updated to check the newly added capability.
>> Existing 'cmd_pause' now only indicates that DMA engine support pausing
>> given DMA channel.
>
> The change looks fine to me. I was hoping that serial would also check
> this..

AFAIU serial does not need to resume.

-- 
With Best Regards,
Andy Shevchenko