linux-kernel - Re: 回复: [PATCH] spi: disable chipselect after complete transfer

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Date:   Thu, 10 Feb 2022 15:57:45 +0000
From:   Mark Brown <broonie@...nel.org>
To:     "Zhou, Yun" <Yun.Zhou@...driver.com>
Cc:     "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Xue, Ying" <Ying.Xue@...driver.com>
Subject: Re: 回复: [PATCH] spi: disable
 chipselect after complete transfer

On Thu, Feb 10, 2022 at 03:41:41PM +0000, Zhou, Yun wrote:

Please don't top post, reply in line with needed context.  This allows
readers to readily follow the flow of conversation and understand what
you are talking about and also helps ensure that everything in the
discussion is being addressed.

> I am sorry that I can not agree with you.

> If there are multiple messages, and each message only has one xfer,
> and the cs_change of each xfer is 1, during the transmission of the
> messages, the CS will keep active even until at the end. This must be
> unreasonable.

This is not something that most drivers are expected to use, cs_change
should only be being used at all for very unusual hardware and it should
be used even less frequently for the last transfer in a message.  It is
fragile and anyone using it really needs to know what they're doing but
the feature is there.

> I can't understand why it have to keep CS active after the
> transmission is completed. Could you please explain this in detail?

The feature predates me working on the SPI stack, the obvious examples
would be a device that doesn't actually use chip select where you want
to avoid all chip select changes or if you need to do some other actions
in the middle of a SPI transaction for some reason (which would need a
bunch of system level considerations to actually be safe/sensible like
making sure you're not sharing the SPI bus).

Please fix your mail client to word wrap within paragraphs at something
substantially less than 80 columns.  Doing this makes your messages much
easier to read and reply to.

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