linux-kernel - Re: [PATCH 1/7] dt-bindings: spi: zynqmp-qspi: Split the bus

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Message-ID: <1a8c923f-0905-4cc0-9fbd-949d29a2f39b@linux.dev>
Date: Fri, 13 Jun 2025 12:44:20 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: David Lechner <dlechner@...libre.com>, Mark Brown <broonie@...nel.org>,
 Michal Simek <michal.simek@....com>, linux-spi@...r.kernel.org
Cc: Jinjie Ruan <ruanjinjie@...wei.com>,
 linux-arm-kernel@...ts.infradead.org,
 Amit Kumar Mahapatra <amit.kumar-mahapatra@....com>,
 linux-kernel@...r.kernel.org, Miquel Raynal <miquel.raynal@...tlin.com>,
 Conor Dooley <conor+dt@...nel.org>, Krzysztof Kozlowski
 <krzk+dt@...nel.org>, Rob Herring <robh@...nel.org>,
 devicetree@...r.kernel.org,
 "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
 Jonathan Cameron <jic23@...nel.org>, Nuno Sá
 <nuno.sa@...log.com>
Subject: Re: [PATCH 1/7] dt-bindings: spi: zynqmp-qspi: Split the bus

On 6/13/25 11:57, Sean Anderson wrote:
> On 6/13/25 10:20, David Lechner wrote:
>> On 6/12/25 6:44 PM, Sean Anderson wrote:
>>> Hi David,
>>> 
>>> I am (finally!) getting around to doing v2 of this series, and I ran
>>> into a small problem with your proposed solution.
>>> 
>>> On 1/23/25 16:59, David Lechner wrote:
>>>> ---
>>>> From: David Lechner <dlechner@...libre.com>
>>>> Date: Thu, 23 Jan 2025 15:35:19 -0600
>>>> Subject: [PATCH 2/2] spi: add support for multi-bus controllers
>>>>
>>>> Add support for SPI controllers with multiple physical SPI buses.
>>>>
>>>> This is common in the type of controller that can be used with parallel
>>>> flash memories, but can be used for general purpose SPI as well.
>>>>
>>>> To indicate support, a controller just needs to set ctlr->num_buses to
>>>> something greater than 1. Peripherals indicate which bus they are
>>>> connected to via device tree (ACPI support can be added if needed).
>>>>
>>>> In the future, this can be extended to support peripherals that also
>>>> have multiple SPI buses to use those buses at the same time by adding
>>>> a similar bus flags field to struct spi_transfer.
>>>>
>>>> Signed-off-by: David Lechner <dlechner@...libre.com>
>>>> ---
>>>>  drivers/spi/spi.c       | 26 +++++++++++++++++++++++++-
>>>>  include/linux/spi/spi.h | 13 +++++++++++++
>>>>  2 files changed, 38 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
>>>> index 10c365e9100a..f7722e5e906d 100644
>>>> --- a/drivers/spi/spi.c
>>>> +++ b/drivers/spi/spi.c
>>>> @@ -2364,7 +2364,7 @@ static void of_spi_parse_dt_cs_delay(struct device_node *nc,
>>>>  static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
>>>>  			   struct device_node *nc)
>>>>  {
>>>> -	u32 value, cs[SPI_CS_CNT_MAX];
>>>> +	u32 value, buses[8], cs[SPI_CS_CNT_MAX];
>>>>  	int rc, idx;
>>>>  
>>>>  	/* Mode (clock phase/polarity/etc.) */
>>>> @@ -2379,6 +2379,29 @@ static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
>>>>  	if (of_property_read_bool(nc, "spi-cs-high"))
>>>>  		spi->mode |= SPI_CS_HIGH;
>>>>  
>>>> +	rc = of_property_read_variable_u32_array(nc, "spi-buses", buses, 1,
>>>> +						 ARRAY_SIZE(buses));
>>>> +	if (rc < 0 && rc != -EINVAL) {
>>>> +		dev_err(&ctlr->dev, "%pOF has invalid 'spi-buses' property (%d)\n",
>>>> +			nc, rc);
>>>> +		return rc;
>>>> +	}
>>>> +
>>>> +	if (rc == -EINVAL) {
>>>> +		/* Default when property is omitted. */
>>>> +		spi->buses = BIT(0);
>>> 
>>> For backwards compatibility, the default bus for CS 1 on gqspi must be 1
>>> and not 0. Ideally there would be some hook for the master to fix things
>>> up when the slaves are probed, but that doesn't seem to exist. I was
>>> thinking about doing this with OF changesets. Do you have any better
>>> ideas?
>>> 
>> 
>> Does this work? 
>> 
>> 		spi->buses = BIT(cs[0]);
>> 
>> (would have to move all the new code after cs[0] is assigned of course)
> 
> Yeah, but do we really want to make this the default for all drivers?
> This is really a quirk of the existing gqspi binding and I don't think
> it makes sense in general.

I think I will add a flag like

		/* Default when property is omitted. */
		if (ctlr->flags & SPI_CONTROLLER_DEFAULT_BUS_IS_CS)
			spi->buses = BIT(cs[0]);
		else
			spi->buses = BIT(0);

which should keep the defaults sane for everyone else.

--Sean