linux-kernel - Re: [PATCH 1/4] spi: For controllers that need realtime always use the pump thread

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Message-ID: <20190512073301.GC21483@sirena.org.uk>
Date:   Sun, 12 May 2019 16:33:01 +0900
From:   Mark Brown <broonie@...nel.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Benson Leung <bleung@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-rockchip@...ts.infradead.org, drinkcat@...omium.org,
        Guenter Roeck <groeck@...omium.org>, briannorris@...omium.org,
        mka@...omium.org, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org
Subject: Re: [PATCH 1/4] spi: For controllers that need realtime always use
 the pump thread

On Fri, May 10, 2019 at 03:34:34PM -0700, Douglas Anderson wrote:
> If a controller specifies that it needs high priority for sending
> messages we should always schedule our transfers on the thread.  If we
> don't do this we'll do the transfer in the caller's context which
> might not be very high priority.

If performance is important you probably also want to avoid the context
thrashing - executing in the calling context is generally a substantial
performance boost.  I can see this causing problems further down the
line when someone else turns up with a different requirement, perhaps in
an application where the caller does actually have a raised priority
themselves and just wanted to make sure that the thread wasn't lower
than they are.  I guess it'd be nice if we could check what priority the
calling thread has and make a decision based on that but there don't
seem to be any facilities for doing that which I can see right now.

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