linux-kernel - Re: [PATCH] spi: mt7621: allow GPIO chip select lines

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Message-ID: <71533474-eb08-438c-b7ec-5f3277c195fc@sirena.org.uk>
Date: Fri, 15 Mar 2024 20:41:21 +0000
From: Mark Brown <broonie@...nel.org>
To: Justin Swartz <justin.swartz@...ingedge.co.za>
Cc: Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] spi: mt7621: allow GPIO chip select lines

On Fri, Mar 15, 2024 at 10:21:53PM +0200, Justin Swartz wrote:
> On 2024-03-15 19:47, Mark Brown wrote:

> > Look at other drivers that support GPIO chip selects?

> Of the 43 drivers (of drivers/spi/*.c) that setup the
> spi_controller's use_gpio_descriptors as true:

>   39 drivers use the transfer_one hook, and
>    4 drivers use the transfer_one_message hook.

> Drivers that use the transfer_one hook benefit from the core
> taking care of chip selection on their behalf.

> Drivers that use the transfer_one_message hook handle chip
> selection on their own, within the function they've pointed
> the hook at.

Oh, this is an old school driver.  Glancing at the code I can't see any
particular reason why it's not using transfer_one(), you should just
convert the driver to that which will reduce the open coding and just
generally improve functionality.  You could add a callback to flush the
write FIFO or add that into the write function, I'm not sure if there's
a meaningful performance benefit there.

> Considering spi-mt7621.c was implemented using the
> transfer_one_message() hook, I'd assumed that it made more

I think it's just old and based on having gone through staging likely
based on even older BSP code.

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