linux-kernel - Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce buffer support

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Message-ID: <20170302170041.hfix33mhzgqydmvh@sirena.org.uk>
Date:   Thu, 2 Mar 2017 17:00:41 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     Vignesh R <vigneshr@...com>, Frode Isaksen <fisaksen@...libre.com>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        Richard Weinberger <richard@....at>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Marek Vasut <marek.vasut@...il.com>,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-omap@...r.kernel.org, linux-spi@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce
 buffer support

On Thu, Mar 02, 2017 at 03:29:21PM +0100, Boris Brezillon wrote:
> Vignesh R <vigneshr@...com> wrote:

> > Or SPI core can be extended in a way similar to this RFC. That is, SPI
> > master driver will set a flag to request SPI core to use of bounce
> > buffer for vmalloc'd buffers. And spi_map_buf() just uses bounce buffer
> > in case buf does not belong to kmalloc region based on the flag.

> That's a better approach IMHO. Note that the decision should not only

I don't understand how the driver is supposed to tell if it might need a
bounce buffer due to where the memory is allocated and the caches used
by the particular system it is used on?  The suggestion to pass via
scatterlists seems a bit more likely to work but even then I'm not clear
that drivers doing PIO would play well.

> be based on the buffer type, but also on the transfer length and/or
> whether the controller supports transferring non physically contiguous
> buffers.

The reason most drivers only look at the transfer length when deciding
that they can DMA is that most controllers are paired with DMA
controllers that are sensibly implemented, the only factor they're
selecting on is the copybreak for performance.

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