linux-kernel - Re: [PATCH 17/20] PCI: dw-rockchip: switch to HWORD

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Message-ID: <aEvzMnxgsjfryCOo@ryzen>
Date: Fri, 13 Jun 2025 11:45:22 +0200
From: Niklas Cassel <cassel@...nel.org>
To: Nicolas Frattaroli <nicolas.frattaroli@...labora.com>
Cc: Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
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	Kishon Vijay Abraham I <kishon@...nel.org>,
	Nicolas Frattaroli <frattaroli.nicolas@...il.com>,
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Subject: Re: [PATCH 17/20] PCI: dw-rockchip: switch to HWORD_UPDATE macro

Hello Nicolas,

On Thu, Jun 12, 2025 at 08:56:19PM +0200, Nicolas Frattaroli wrote:
> 
> PCIE_CLIENT_RC_MODE/PCIE_CLIENT_EP_MODE was another field that wasn't
> super clear on what the bit field modification actually is. As far as I
> can tell, switching to RC mode doesn't actually write the correct value
> to the field if any of its bits have been set previously, as it only
> updates one bit of a 4 bit field.
> 
> Replace it by actually writing the full values to the field, using the
> new HWORD_UPDATE macro, which grants us the benefit of better
> compile-time error checking.

The current code looks like this:
#define  PCIE_CLIENT_RC_MODE            HIWORD_UPDATE_BIT(0x40)
#define  PCIE_CLIENT_EP_MODE            HIWORD_UPDATE(0xf0, 0x0)

The device_type field is defined like this:
4'h0: PCI Express endpoint
4'h1: Legacy PCI Express endpoint
4'h4: Root port of PCI Express root complex

The reset value of the device_type field is 0x0 (EP mode).

So switching between RC mode / EP mode should be fine.

But I agree, theoretically there could be a bug if e.g. bootloader
has set the device_type to 0x1 (Legacy EP).

So if you want, you could send a patch:
-#define  PCIE_CLIENT_RC_MODE            HIWORD_UPDATE_BIT(0x40)
+#define  PCIE_CLIENT_RC_MODE            HIWORD_UPDATE(0xf0, 0x40)

With:
Fixes: 0e898eb8df4e ("PCI: rockchip-dwc: Add Rockchip RK356X host controller driver")

But I also think that your current patch is fine as-is.

I do however think that you can drop this line:
+#define  PCIE_CLIENT_MODE_LEGACY       0x1U

Since the define is never used.


Also, is there any point in adding the U suffix?

Usually you see UL or ULL suffix, when that is needed, but there actually
seems to be extremely few hits of simply U suffix:
$ git grep 0x1U | grep -v UL


Kind regards,
Niklas