[PATCH v1] spi: designware: add support for bits-per-word DT binding

[PATCH v1] spi: designware: add support for bits-per-word DT binding

[Boon Khai Ng]

Add support for the bits-per-word device tree binding.

Signed-off-by: Boon Khai Ng <boon.khai.ng@altera.com>
---
 drivers/spi/designware_spi.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/spi/designware_spi.c b/drivers/spi/designware_spi.c
index b520c727900..c41436a1dfb 100644
--- a/drivers/spi/designware_spi.c
+++ b/drivers/spi/designware_spi.c
@@ -359,6 +359,7 @@ static int dw_spi_probe(struct udevice *bus)
 	dw_spi_init_t init = (dw_spi_init_t)dev_get_driver_data(bus);
 	struct dw_spi_plat *plat = dev_get_plat(bus);
 	struct dw_spi_priv *priv = dev_get_priv(bus);
+	u32 bits_per_word = 8;
 	int ret;
 	u32 version;
 
@@ -384,8 +385,19 @@ static int dw_spi_probe(struct udevice *bus)
 		version >> 24, version >> 16, version >> 8, version,
 		priv->max_xfer);
 
-	/* Currently only bits_per_word == 8 supported */
-	priv->bits_per_word = 8;
+	ret = ofnode_read_u32(dev_ofnode(bus), "bits-per-word", &bits_per_word);
+	if (ret) {
+		debug("%s no bits-per-word not found\n", __func__);
+	} else {
+		if (bits_per_word >= 4 && bits_per_word <= priv->max_xfer) {
+			debug("%s %d bits-per-word found\n", __func__, bits_per_word);
+		} else {
+			printf("ignoring invalid bits-per-word %d\n", bits_per_word);
+			bits_per_word = 8;
+		}
+	}
+
+	priv->bits_per_word = bits_per_word;
 
 	priv->tmode = 0; /* Tx & Rx */
 
-- 
2.43.7

Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Tom Rini]

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On Fri, Feb 27, 2026 at 07:04:34PM +0800, Boon Khai Ng wrote:

> Add support for the bits-per-word device tree binding.
> 
> Signed-off-by: Boon Khai Ng <boon.khai.ng@altera.com>
> ---
>  drivers/spi/designware_spi.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/spi/designware_spi.c b/drivers/spi/designware_spi.c
> index b520c727900..c41436a1dfb 100644
> --- a/drivers/spi/designware_spi.c
> +++ b/drivers/spi/designware_spi.c
> @@ -359,6 +359,7 @@ static int dw_spi_probe(struct udevice *bus)
>  	dw_spi_init_t init = (dw_spi_init_t)dev_get_driver_data(bus);
>  	struct dw_spi_plat *plat = dev_get_plat(bus);
>  	struct dw_spi_priv *priv = dev_get_priv(bus);
> +	u32 bits_per_word = 8;
>  	int ret;
>  	u32 version;
>  
> @@ -384,8 +385,19 @@ static int dw_spi_probe(struct udevice *bus)
>  		version >> 24, version >> 16, version >> 8, version,
>  		priv->max_xfer);
>  
> -	/* Currently only bits_per_word == 8 supported */
> -	priv->bits_per_word = 8;
> +	ret = ofnode_read_u32(dev_ofnode(bus), "bits-per-word", &bits_per_word);
> +	if (ret) {
> +		debug("%s no bits-per-word not found\n", __func__);
> +	} else {
> +		if (bits_per_word >= 4 && bits_per_word <= priv->max_xfer) {
> +			debug("%s %d bits-per-word found\n", __func__, bits_per_word);
> +		} else {
> +			printf("ignoring invalid bits-per-word %d\n", bits_per_word);
> +			bits_per_word = 8;
> +		}
> +	}
> +
> +	priv->bits_per_word = bits_per_word;
>  
>  	priv->tmode = 0; /* Tx & Rx */
>  

I don't see this binding in v7.0-rc1, did I miss it? Thanks.

-- 
Tom

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Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Ng, Boon Khai]

Hi Tom,

> I don't see this binding in v7.0-rc1, did I miss it? Thanks.
> 

This binding was not introduce in Linux. This is a per-requisite commit
to support the Agilex Smart NIC 60xx board 
https://www.silicom-usa.com/wp-content/uploads/2024/02/FPGA-SmartNIC-N6010-AgileX-Based.pdf

During the initialization stage, this board require a spi communication 
to configure the chip's PLL and also to check the chip's status.

and it require the specific spi bits-per-word=16 setting.

Can i add the binding at U-Boot?
https://github.com/u-boot/u-boot/blob/master/doc/device-tree-bindings/spi/snps%2Cdw-apb-ssi.txt

Regards
Boon Khai.

Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Tom Rini]

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On Tue, Mar 03, 2026 at 03:59:35PM +0800, Ng, Boon Khai wrote:
> Hi Tom,
> 
> > I don't see this binding in v7.0-rc1, did I miss it? Thanks.
> > 
> 
> This binding was not introduce in Linux. This is a per-requisite commit
> to support the Agilex Smart NIC 60xx board https://www.silicom-usa.com/wp-content/uploads/2024/02/FPGA-SmartNIC-N6010-AgileX-Based.pdf
> 
> During the initialization stage, this board require a spi communication to
> configure the chip's PLL and also to check the chip's status.
> 
> and it require the specific spi bits-per-word=16 setting.
> 
> Can i add the binding at U-Boot?
> https://github.com/u-boot/u-boot/blob/master/doc/device-tree-bindings/spi/snps%2Cdw-apb-ssi.txt

Why would this be inappropraite for the Linux kernel, once the system is
fully supported in Linux is the first question to answer. Thanks.

-- 
Tom

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Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Conor Dooley]

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On Tue, Mar 03, 2026 at 08:37:21AM -0600, Tom Rini wrote:
> On Tue, Mar 03, 2026 at 03:59:35PM +0800, Ng, Boon Khai wrote:
> > Hi Tom,
> > 
> > > I don't see this binding in v7.0-rc1, did I miss it? Thanks.
> > > 
> > 
> > This binding was not introduce in Linux. This is a per-requisite commit
> > to support the Agilex Smart NIC 60xx board https://www.silicom-usa.com/wp-content/uploads/2024/02/FPGA-SmartNIC-N6010-AgileX-Based.pdf
> > 
> > During the initialization stage, this board require a spi communication to
> > configure the chip's PLL and also to check the chip's status.
> > 
> > and it require the specific spi bits-per-word=16 setting.
> > 
> > Can i add the binding at U-Boot?
> > https://github.com/u-boot/u-boot/blob/master/doc/device-tree-bindings/spi/snps%2Cdw-apb-ssi.txt
> 
> Why would this be inappropraite for the Linux kernel, once the system is

Did you mean "inappropraite", or actually appropriate? Guess it
doesn't really matter to me, since the former would be asking "why would
linux not want this" and the latter would be "why would linux want
this". And I guess, as U-Boot maintainer the former is more important so
inappropriate is actually what you meant. I, of course, always approach
things with the latter mindset ;)

> fully supported in Linux is the first question to answer. Thanks.

The property doesn't make sense to me in a linux context. bits_per_word
as explained here is a device setting should be set in the device driver,
and is almost always per-compatible so doesn't even need to come from dt
at all. What a controller can support is also effectively always
determinable from the compatible of the controller, and is often a range.

Looking at the linux driver, all dw spi controllers appear to be able to
support 4-16 bits per word with select devices also supporting 32. The
check is done at runtime, it has no need for a property at all. The
device driver effectively decides what is done for the transfer, within
the range of supported values by the controller, based on what it sets
in its spi_device struct. I don't know if u-boot has similar
functionality in its code, but looking at dm_spi_slave_plat which I
think is your version of spi_device, that information is not there.

Were this submitted to linux with the current explanation, the feedback
would be to set bits_per_word in the driver for whatever this "chip"
that's being talked about is, presumably that means setting it in some
sort of clock driver given that it controls a PLL?

Cheers,
Conor.

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Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Ng, Boon Khai]

Hi Conor, Tom,

>> Why would this be inappropraite for the Linux kernel, once the system is
>> fully supported in Linux is the first question to answer. Thanks.

As Conor mentioned, the bits_per_word setting is obtained from the 
controller and validated at runtime.

Additionally, bits_per_word can also be sourced from ACPI, as shown in 
the Linux SPI driver code

https://github.com/torvalds/linux/blob/af4e9ef3d78420feb8fe58cd9a1ab80c501b3c08/drivers/spi/spi.c#L2739C7-L2739C20

> in its spi_device struct. I don't know if u-boot has similar
> functionality in its code, but looking at dm_spi_slave_plat which I
> think is your version of spi_device, that information is not there.
> 
Currently, U-Boot only supports an hardcoded 8-bit bits_per_word 
configuration.

> Were this submitted to linux with the current explanation, the feedback
> would be to set bits_per_word in the driver for whatever this "chip"
> that's being talked about is, presumably that means setting it in some
> sort of clock driver given that it controls a PLL?
> 
No, this isn't required by Linux, as the PLL only needs to be configured 
during the initial stage. This commit is specifically necessary for 
supporting the Agilex Smart NIC board, and I'm currently preparing 
patches for upstream submission. More details on the chip can be found, 
https://www.microchip.com/en-us/product/zl30793

Below is the link to the Smart NIC board:
https://www.silicom-usa.com/pr/4g-5g-products/4g-5g-adapters/fpga-smartnic-n6010-intel-based/

Below is the link to the patches I am preparing for upstream submission:
https://github.com/altera-fpga/u-boot-socfpga/blob/socfpga_v2025.10/board/intel/agilex-n6010/plldata.c

-- 
 >

Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Tom Rini]

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On Wed, Mar 04, 2026 at 01:00:13AM +0800, Ng, Boon Khai wrote:
> Hi Conor, Tom,
> 
> > > Why would this be inappropraite for the Linux kernel, once the system is
> > > fully supported in Linux is the first question to answer. Thanks.
> 
> As Conor mentioned, the bits_per_word setting is obtained from the
> controller and validated at runtime.
> 
> Additionally, bits_per_word can also be sourced from ACPI, as shown in the
> Linux SPI driver code
> 
> https://github.com/torvalds/linux/blob/af4e9ef3d78420feb8fe58cd9a1ab80c501b3c08/drivers/spi/spi.c#L2739C7-L2739C20
> 
> > in its spi_device struct. I don't know if u-boot has similar
> > functionality in its code, but looking at dm_spi_slave_plat which I
> > think is your version of spi_device, that information is not there.
> > 
> Currently, U-Boot only supports an hardcoded 8-bit bits_per_word
> configuration.

Thanks. And can we reasonably follow what Linux does here instead?

-- 
Tom

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Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Ng, Boon Khai]

> 
> Thanks. And can we reasonably follow what Linux does here instead?
> 

We don't have ACPI in our devices, instead, i think that can be replaced 
with DTS.

-- 
 >

Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Tom Rini]

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On Wed, Mar 04, 2026 at 10:41:57AM +0800, Ng, Boon Khai wrote:
> 
> > 
> > Thanks. And can we reasonably follow what Linux does here instead?
> > 
> 
> We don't have ACPI in our devices, instead, i think that can be replaced
> with DTS.

I thought Conor said it could be auto-detected still, without ACPI, in
Linux?

-- 
Tom

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Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Conor Dooley]

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On Wed, Mar 04, 2026 at 08:38:17AM -0600, Tom Rini wrote:
> On Wed, Mar 04, 2026 at 10:41:57AM +0800, Ng, Boon Khai wrote:
> > 
> > > 
> > > Thanks. And can we reasonably follow what Linux does here instead?
> > > 
> > 
> > We don't have ACPI in our devices, instead, i think that can be replaced
> > with DTS.
> 
> I thought Conor said it could be auto-detected still, without ACPI, in
> Linux?

The controller capabilities are auto-detected. This has nothing to do
with ACPI AFAICT, I don't know why that was brought up. What it detects
is not an exact value though, the detected capability is the max of a
range. On Linux, the spi controller driver advertises the range it
supports. The spi device struct carries the bits_per_word that it wants
to use, and the controller driver looks into that struct to figure out
what supported bits_per_word setting to use in the transfer function.
It's not as simple as just reading the register in the controller and
setting the U-Boot drivers bits_per_word to the value, but I don't see
why U-Boot couldn't do what Linux does here.

If you search in spi-dw-core for bits_per_word, you should be able to
get a grasp of how it works.

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Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Ng, Boon Khai]

Hi, Tom, Conor,
>> configuration.
> 
> Thanks. And can we reasonably follow what Linux does here instead?
> 

After study the linux code, the bits_per_word value can be set at the 
higher level stack for example the touch screen driver ad7877.c at line 
https://github.com/torvalds/linux/blob/1f318b96cc84d7c2ab792fcc0bfd42a7ca890681/drivers/input/touchscreen/ad7877.c#L694

will implement something like that in U-Boot too as follow:
1) Add the bits_per_word in the spi_slave struct
2) at the dw_spi_xfer function to resolve for the slave->bits_per_word 
value and store into the priv->bits_per_word, default to 8 otherwise

Regards,
Boon Khai

-- 
 >

Re: [PATCH v1] spi: designware: add support for bits-per-word DT binding

[Ng, Boon Khai]

> will implement something like that in U-Boot too as follow:
> 1) Add the bits_per_word in the spi_slave struct
> 2) at the dw_spi_xfer function to resolve for the slave->bits_per_word 
> value and store into the priv->bits_per_word, default to 8 otherwise
> 

Submitted the change at:
https://patchwork.ozlabs.org/project/uboot/patch/20260312044633.29242-1-boon.khai.ng@altera.com/

-- 
 >