linux-kernel - Re: [PATCH] PCI: Disallow retraining link for Atheros QCA98xx chips on non-Gen1 PCIe bridges

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Message-ID: <YF540gjh156QIirA@rocinante>
Date:   Sat, 27 Mar 2021 01:14:10 +0100
From:   Krzysztof Wilczyński <kw@...ux.com>
To:     Pali Rohár <pali@...nel.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Marek Behún <kabel@...nel.org>,
        vtolkm@...il.com, Rob Herring <robh@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Jason Cooper <jason@...edaemon.net>, linux-pci@...r.kernel.org,
        ath10k@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: Disallow retraining link for Atheros QCA98xx chips
 on non-Gen1 PCIe bridges

Hi Pali,

Thank you for sending the patch over!

[...]
> +static int pcie_change_tls_to_gen1(struct pci_dev *parent)

Just a nitpick, so feel free to ignore it.  I would just call the
variable "dev" as we pass a pointer to a particular device, but it does
not matter as much, so I am leaving this to you.

[...]
> +	if (ret == 0) {

You prefer this style over "if (!ret)"?  Just asking in the view of the
style that seem to be preferred in the code base at the moment.

> +		/* Verify that new value was really set */
> +		pcie_capability_read_word(parent, PCI_EXP_LNKCTL2, &reg16);
> +		if ((reg16 & PCI_EXP_LNKCTL2_TLS) != PCI_EXP_LNKCTL2_TLS_2_5GT)
> +			ret = -EINVAL;

I am wondering about this verification - did you have a case where the
device would not properly set its capability, or accept the write and do
nothing?

> +	if (ret != 0)

I think "if (ret)" would be fine to use here, unless you prefer being
more explicit.  See my question about style above.

>  static bool pcie_retrain_link(struct pcie_link_state *link)
>  {
>  	struct pci_dev *parent = link->pdev;
>  	unsigned long end_jiffies;
>  	u16 reg16;
> +	u32 reg32;
> +
> +		/* Check if link is capable of higher speed than 2.5 GT/s and needs quirk */
> +		pcie_capability_read_dword(parent, PCI_EXP_LNKCAP, &reg32);
> +		if ((reg32 & PCI_EXP_LNKCAP_SLS) > PCI_EXP_LNKCAP_SLS_2_5GB) {

I wonder if moving this check to pcie_change_tls_to_gen1() would make
more sense?  It would then make this function a little cleaner.  What do
you think?

[...]
> +static void quirk_no_bus_reset_and_no_retrain_link(struct pci_dev *dev)
> +{
> +	dev->dev_flags |= PCI_DEV_FLAGS_NO_BUS_RESET | PCI_DEV_FLAGS_NO_RETRAIN_LINK_WHEN_NOT_GEN1;
> +}
[...]

I know that the style has been changed to allow 100 characters width and
that checkpatch.pl now also does not warn about line length, as per
commit bdc48fa11e46 ("checkpatch/coding-style: deprecate 80-column
warning"), but I think Bjorn still prefers 80 characters, thus this line
above might have to be aligned.

Krzysztof