netdev - Re: [PATCH v3 net-next 1/2] net: dsa: sja1105: send multiple spi_messages instead of using cs

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Message-ID: <20210524130212.g6jcf7y4grc64mki@skbuf>
Date:   Mon, 24 May 2021 16:02:12 +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 v3 net-next 1/2] net: dsa: sja1105: send multiple
 spi_messages instead of using cs_change

On Mon, May 24, 2021 at 09:35:29AM +0100, Mark Brown wrote:
> On Fri, May 21, 2021 at 12:16:56AM +0300, Vladimir Oltean wrote:
> 
> > The fact of the matter is that spi_max_message_size() has an ambiguous
> > meaning if any non-final transfer has cs_change = true.
> 
> This is not the case, spi_message_max_size() is a limit on the size of a
> spi_message.

That is true, although it doesn't mean much, since in the presence of
cs_change, a spi_message has no correspondent in the physical world
(i.e. you can't look at a logic analyzer dump and say "this spi_message
was from this to this point"), and that is the problem really.
Describing the controller's inability to send more than N SPI words with
continuous chip select using spi_message_max_size() is what seems flawed
to me, but it's what we have, and what I've adapted to.