netdev - Re: [PATCH net-next] net: dsa: sja1105: adapt to a SPI controller with a limited max transfer size

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Date:   Thu, 20 May 2021 17:06:09 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        linux-spi@...r.kernel.org, Guenter Roeck <linux@...ck-us.net>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next] net: dsa: sja1105: adapt to a SPI controller
 with a limited max transfer size

Hi Mark,

On Thu, May 20, 2021 at 02:56:15PM +0100, Mark Brown wrote:
> On Thu, May 20, 2021 at 04:50:31PM +0300, Vladimir Oltean wrote:
> 
> > Only that certain SPI controllers, such as the spi-sc18is602 I2C-to-SPI
> > bridge, cannot keep the chip select asserted for that long.
> > The spi_max_transfer_size() and spi_max_message_size() functions are how
> > the controller can impose its hardware limitations upon the SPI
> > peripheral driver.
> 
> You should respect both, frankly I don't see any advantage to using
> cs_change for something like this - just do a bunch of async SPI
> transfers and you'll get the same effect in terms of being able to keep
> the queue for the controller primed with more robust support since it's
> not stressing edge cases.  cs_change is more for doing things that are
> just very non-standard.

Sorry, I don't really understand your comment: in which way would it be
more robust for my use case to use spi_async()?

The cs_change logic was already there prior to this patch, I am just
reiterating how it works. Given the way in which it works (which I think
is correct), the most natural way to limit the buffer length is to look
for the max transfer len.