linux-kernel - Re: [PATCH v2] arm64: dts: qcom: x1e80100: Switch PCIe 6a to 4 lanes mode

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Message-ID: <ZwPDxd9JJbgDeJTi@hovoldconsulting.com>
Date: Mon, 7 Oct 2024 13:19:33 +0200
From: Johan Hovold <johan@...nel.org>
To: Abel Vesa <abel.vesa@...aro.org>
Cc: Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konradybcio@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
	linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: dts: qcom: x1e80100: Switch PCIe 6a to 4 lanes
 mode

On Fri, Oct 04, 2024 at 12:06:33PM +0300, Abel Vesa wrote:
> The PCIe 6a controller and PHY can be configured in 4-lanes mode or
> 2-lanes mode. For 4-lanes mode, it fetches the lanes provided by PCIe 6b.

nit: I still think you should use "uses" over "fetches" here.

> For 2-lanes mode, PCIe 6a uses 2 lanes and then PCIe 6b uses the other 2
> lanes. Configure it in 4-lane mode and then each board can configure it
> depending on the design. 

To avoid confusion you could avoid the word "configure" here. pcie6a is
a four-lane controller (and PHY) that can also be used in used in
two-lane mode, depending on how the system is configured and this can be
read out through a TCSR register (e.g. the PHY driver adapts
accordingly).

So you should perhaps rather say that you are fixing the description and
compatible of pcie6a, which *is* a 4-lane controller, that can also be
used in 2-lane mode. Or similar.

We also discussed this here:

	https://lore.kernel.org/lkml/ZtG2dUVkdwBpBbix@hovoldconsulting.com/

> Both the QCP and CRD boards, currently upstream,
> use PCIe 6a for NVMe in 4-lane mode. Mark the controller as 4-lane as
> well.

> This is the last change needed in order to support NVMe with Gen4
> 4-lanes on all existing X Elite boards.

I'd skip comments like this, or put them in the cover letter, or just
rephrase as "This will enable 4-lane NVMe on...".

> Signed-off-by: Abel Vesa <abel.vesa@...aro.org>
> ---
> Changes in v2:
> - Re-worded the commit message according to Johan's suggestions
> - Dropped the clocks changes.
> - Dropped the fixes tag as this relies on the Gen4 4-lanes stability
>   patchset which has been only merged in 6.12, so backporting this patch
>   would break NVMe support for all platforms.
> - Link to v1: https://lore.kernel.org/r/20240531-x1e80100-dts-fixes-pcie6a-v1-2-1573ebcae1e8@linaro.org

Patch itself looks good.

Johan