linux-kernel - Re: [PATCH] [RFC] dmaengine: add fifo

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Date:   Thu, 6 Jun 2019 17:32:37 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Sameer Pujar <spujar@...dia.com>, Vinod Koul <vkoul@...nel.org>
CC:     <dan.j.williams@...el.com>, <tiwai@...e.com>,
        <dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <sharadg@...dia.com>, <rlokhande@...dia.com>, <dramesh@...dia.com>,
        <mkumard@...dia.com>, linux-tegra <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH] [RFC] dmaengine: add fifo_size member


On 06/06/2019 16:18, Dmitry Osipenko wrote:

...

>>> If I understood everything correctly, the FIFO buffer is shared among
>>> all of the ADMA clients and hence it should be up to the ADMA driver to
>>> manage the quotas of the clients. So if there is only one client that
>>> uses ADMA at a time, then this client will get a whole FIFO buffer, but
>>> once another client starts to use ADMA, then the ADMA driver will have
>>> to reconfigure hardware to split the quotas.
>>
>> The FIFO quotas are managed by the ADMAIF driver (does not exist in
>> mainline currently but we are working to upstream this) because it is
>> this device that owns and needs to configure the FIFOs. So it is really
>> a means to pass the information from the ADMAIF to the ADMA.
> 
> So you'd want to reserve a larger FIFO for an audio channel that has a
> higher audio rate since it will perform reads more often. You could also
> prioritize one channel over the others, like in a case of audio call for
> example.
> 
> Is the shared buffer smaller than may be needed by clients in a worst
> case scenario? If you could split the quotas statically such that each
> client won't ever starve, then seems there is no much need in the
> dynamic configuration.

Actually, this is still very much relevant for the static case. Even if
we defined a static configuration of the FIFO mapping in the ADMAIF
driver we still need to pass this information to the ADMA. I don't
really like the idea of having it statically defined in two different
drivers.

Jon

-- 
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