linux-kernel - Re: [PATCH] can: mcp251x: convert to half-duplex SPI

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Message-ID: <a663d610-56d8-1c2d-90ad-4d128a2fa3a8@pengutronix.de>
Date:   Mon, 25 May 2020 15:12:05 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Mark Brown <broonie@...nel.org>
Cc:     Tim Harvey <tharvey@...eworks.com>,
        open list <linux-kernel@...r.kernel.org>,
        linux-can@...r.kernel.org, Wolfgang Grandegger <wg@...ndegger.com>,
        Timo Schlüßler <schluessler@...use.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-spi@...r.kernel.org, Jan Glauber <jglauber@...vell.com>,
        Robert Richter <rrichter@...vell.com>
Subject: Re: [PATCH] can: mcp251x: convert to half-duplex SPI

On 5/25/20 2:57 PM, Mark Brown wrote:
> On Mon, May 25, 2020 at 02:41:31PM +0200, Marc Kleine-Budde wrote:
>> On 5/25/20 1:31 PM, Mark Brown wrote:
> 
>>> This isn't something that every individual driver should be doing, such
>>> rewriting should happen in the core so that everything sees the benefit.
> 
>> The core could merge several half duplex transfers (until there's as cs_change)
>> into a single full duplex transfer.
> 
> Yes, that is what I am suggesting.

Where in the SPI stack do you see such a "merge" function? One point to clarify
is when and where to allocate and free the memory for the contiguous full duplex
buffers.

>> I think it's not easy to detect and reliable to split a full duplex transfer
>> into half duplex ones. How can you tell, if the controller is supposed to tx 0x0
>> or actually receive.
> 
> I don't understand how that could possibly work or why it would make
> sense?

ACK, I was just thinking loud about options.

>> I think spi_write_then_read() can be extended to generate one full duplex
>> transfer instead on two half duplex ones it does a memcpy() anyways.
> 
> This has the same problem as doing it in any other driver code - it
> causes a needless incompatibility with three wire and single duplex
> devices.  

What about the note "portable code should never use this for more than 32 bytes"
in spi_write_then_read()? The CAN driver in question may read more than 32 bytes
of data.

>> To get a feeling for the use cases, this is what I do in the regmap read
>> function of a (not yet mainlined) CAN SPI driver.
> 
> Like I say it's probably better if code like this gets pushed into the
> SPI core where we've got more information about what the controller can
> do and there's more win from doing the tuning since more devices and
> systems can take advantage of it.

ACK

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
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