linux-kernel - Re: [PATCH v2 2/2] mwifiex: Try waking the firmware until we get an interrupt

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Date:   Mon, 4 Oct 2021 10:52:11 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Jonas Dreßler <verdre@...d.nl>
Cc:     Amitkumar Karwar <amitkarwar@...il.com>,
        Ganapathi Bhat <ganapathi017@...il.com>,
        Xinming Hu <huxinming820@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Tsuchiya Yuto <kitakar@...il.com>,
        Linux Wireless <linux-wireless@...r.kernel.org>,
        "open list:NETWORKING DRIVERS" <netdev@...r.kernel.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Maximilian Luz <luzmaximilian@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Pali Rohár <pali@...nel.org>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Johannes Berg <johannes@...solutions.net>,
        "# 9798ac6d32c1 mfd : cros_ec : Add cros_ec_cmd_xfer_status helper" 
        <stable@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] mwifiex: Try waking the firmware until we get an interrupt

Hi,

On Sun, Oct 3, 2021 at 2:18 AM Jonas Dreßler <verdre@...d.nl> wrote:
> So I think I have another solution that might be a lot more elegant, how
> about this:
>
> try_again:
>         n_tries++;
>
>         mwifiex_write_reg(adapter, reg->fw_status, FIRMWARE_READY_PCIE);
>
>         if (wait_event_interruptible_timeout(adapter->card_wakeup_wait_q,
>                                              READ_ONCE(adapter->int_status) != 0,
>                                              WAKEUP_TRY_AGAIN_TIMEOUT) == 0 &&
>             n_tries < MAX_N_WAKEUP_TRIES) {
>                 goto try_again;
>         }

Isn't wait_event_interruptible_timeout()'s timeout in jiffies, which
is not necessarily that predictable, and also a lot more
coarse-grained than we want? (As in, if HZ=100, we're looking at
precision on the order of 10ms, whereas the expected wakeup latency is
~6ms.) That would be OK for well-behaved PCI cases, where we never
miss a write, but it could ~double your latency for your bad systems
that will need more than one run of the loop.

Also, feels like a do/while could be cleaner, but that's a lesser detail.

> and then call wake_up_interruptible() in the mwifiex_interrupt_status()
> interrupt handler.
>
> This solution should make sure we always keep wakeup latency to a minimum
> and can still retry the register write if things didn't work.

Brian