linux-kernel - Re: [PATCH 5.10 126/599] PCI: pciehp: Clear cmd

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Message-ID: <20220409081314.GA19452@amd>
Date:   Sat, 9 Apr 2022 10:13:15 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Liguang Zhang <zhangliguang@...ux.alibaba.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lukas Wunner <lukas@...ner.de>
Subject: Re: [PATCH 5.10 126/599] PCI: pciehp: Clear cmd_busy bit in polling
 mode

Hi!

> From: Liguang Zhang <zhangliguang@...ux.alibaba.com>
> 
> commit 92912b175178c7e895f5e5e9f1e30ac30319162b upstream.
> 
> Writes to a Downstream Port's Slot Control register are PCIe hotplug
> "commands."  If the Port supports Command Completed events, software must
> wait for a command to complete before writing to Slot Control again.
> 
> pcie_do_write_cmd() sets ctrl->cmd_busy when it writes to Slot Control.  If
> software notification is enabled, i.e., PCI_EXP_SLTCTL_HPIE and
> PCI_EXP_SLTCTL_CCIE are set, ctrl->cmd_busy is cleared by pciehp_isr().
> 
> But when software notification is disabled, as it is when pcie_init()
> powers off an empty slot, pcie_wait_cmd() uses pcie_poll_cmd() to poll for
> command completion, and it neglects to clear ctrl->cmd_busy, which leads to
> spurious timeouts:

I'm pretty sure this fixes the problem, but... it is still not fully
correct.

> +++ b/drivers/pci/hotplug/pciehp_hpc.c
> @@ -98,6 +98,8 @@ static int pcie_poll_cmd(struct controll
>  		if (slot_status & PCI_EXP_SLTSTA_CC) {
>  			pcie_capability_write_word(pdev, PCI_EXP_SLTSTA,
>  						   PCI_EXP_SLTSTA_CC);
> +			ctrl->cmd_busy = 0;
> +			smp_mb();
>  			return 1;
>  		}

Is the memory barrier neccessary? I don't see corresponding memory
barrier for reading.

If it is neccessary, should we have WRITE_ONCE at the very least, or
probably normal atomic operations?

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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