From: Yun Zhou <yun.zhou@windriver.com>
To: Mark Brown <broonie@kernel.org>
Cc: linux-spi@vger.kernel.org, linux-kernel@vger.kernel.org,
ying.xue@windriver.com
Subject: Re: [PATCH] spi: disable chipselect after complete transfer
Date: Thu, 10 Feb 2022 10:03:16 +0800 [thread overview]
Message-ID: <07aca74d-c74e-c2a3-b8eb-bdcf9ed21a55@windriver.com> (raw)
In-Reply-To: <YgPEWyEj7lbQJgAn@sirena.org.uk>
Hi Brown,
Nice to get feedback from you!
In current source code of spi_transfer_one_message(),
1420 bool keep_cs = false;
1488 if (xfer->cs_change) {
1489 if (list_is_last(&xfer->transfer_list,
1490 &msg->transfers)) {
1491 keep_cs = true;
1492 } else {
1493 spi_set_cs(msg->spi, false, false);
1494 _spi_transfer_cs_change_delay(msg, xfer);
1495 spi_set_cs(msg->spi, true, false);
1496 }
1497 }
1502 out:
1503 if (ret != 0 || !keep_cs)
1504 spi_set_cs(msg->spi, false, false);
if the last xfer->cs_change is true, keep_cs will be true, and it will
not call spi_set_cs() to set CS to false. Do you mean to keep CS enabled
in this case?
On 2/9/22 9:40 PM, Mark Brown wrote:
> On Wed, Feb 09, 2022 at 06:00:42PM +0800, Yun Zhou wrote:
>> If there are 2 slaves or more on a spi bus, e.g. A and B, we processed a
>> transfer to A, the CS will be selected for A whose 'last_cs_enable' will
>> be recorded to true at the same time. Then we processed a transfer to B,
>> the CS will be switched to B. And then if we transmit data to A again, it
>> will not enable CS back to A because 'last_cs_enable' is true.
>> In addition, if CS is not disabled, Some controllers in automatic
>> transmission state will receive unpredictable data, such as Cadence SPI
>> controller.
> This sounds like you've got an issue with mixing devices with and
> without CS_HIGH - that is probably broken but...
>
>> out:
>> - if (ret != 0 || !keep_cs)
>> - spi_set_cs(msg->spi, false, false);
>> + spi_set_cs(msg->spi, false, false);
> ...this will obviously break cs_change support, clearly that's not OK.
> The last_cs_high should be moved to the device.
I do not think it will break cs_change support. In my understanding,
cs_change indicates whether or not change CS after an xfer completed.
But at present if the last xfer->cs_change is true, we will not change
CS to disabled state. Is this the result we want? I'm confused.
I look forward to your help and explanation.
Regards,
Yun
next prev parent reply other threads:[~2022-02-10 2:09 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-09 10:00 [PATCH] spi: disable chipselect after complete transfer Yun Zhou
2022-02-09 13:40 ` Mark Brown
2022-02-10 2:03 ` Yun Zhou [this message]
2022-02-10 11:18 ` Mark Brown
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