linux-kernel - Re: [PATCH v2 1/9] platform/surface: Add Surface Aggregator subsystem

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Message-ID: <9748d778-b5e9-c80c-5968-a77b3203d769@redhat.com>
Date:   Tue, 8 Dec 2020 15:43:31 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Maximilian Luz <luzmaximilian@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Mark Gross <mgross@...ux.intel.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Barnabás Pőcze <pobrn@...tonmail.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Blaž Hrastnik <blaz@...n.io>,
        Dorian Stoll <dorian.stoll@...p.io>,
        platform-driver-x86@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-acpi@...r.kernel.org
Subject: Re: [PATCH v2 1/9] platform/surface: Add Surface Aggregator subsystem

Hi,

On 12/8/20 3:37 PM, Maximilian Luz wrote:

<snip>

>>> +
>>> +    obj = acpi_evaluate_dsm_typed(handle, &SSAM_SSH_DSM_GUID,
>>> +                      SSAM_SSH_DSM_REVISION, func, NULL,
>>> +                      ACPI_TYPE_INTEGER);
>>> +    if (!obj)
>>> +        return -EIO;
>>> +
>>> +    val = obj->integer.value;
>>> +    ACPI_FREE(obj);
>>> +
>>> +    if (val > U32_MAX)
>>> +        return -ERANGE;
>>> +
>>> +    *ret = val;
>>> +    return 0;
>>> +}
> 
> [...]
> 
>>> +/**
>>> + * ssam_controller_start() - Start the receiver and transmitter threads of the
>>> + * controller.
>>> + * @ctrl: The controller.
>>> + *
>>> + * Note: When this function is called, the controller should be properly
>>> + * hooked up to the serdev core via &struct serdev_device_ops. Please refer
>>> + * to ssam_controller_init() for more details on controller initialization.
>>> + *
>>> + * This function must be called from an exclusive context with regards to the
>>> + * state, if necessary, by locking the controller via ssam_controller_lock().
>>
>> Again you are being a bit hand-wavy (I assume you know what I mean by that)
>> wrt the locking requirements. If possible I would prefer clearly spelled out
>> locking requirements in the form of "this and that lock must be held when
>> calling this function". Preferably backed-up by lockdep_assert-s asserting
>> these conditions.
> 
> The reason for this is that this function specifically is currently only
> called during initialization, when the controller has not been published
> yet, i.e. when we have an exclusive reference to the controller.
> 
> I'll change this to fully enforce locking (with lockdep_assert).
> 
>> And maybe if you are a bit stricter with always holding the lock when
>> calling this, you can also drop the WRITE_ONCE and the comment about it
>> (in all places where you do this).
> 
> The WRITE_ONCE is only there to ensure that the basic test in
> ssam_request_sync_submit() can be done. I always try to be explicit
> about access that can happen without the respective locks being held.

Yes I saw the matching READ_ONCE later on (as the comment indicated
I would), which made it more obvious to me why the WRITE_ONCE is here,'
so maybe I should have gone back and updated this comment.

Anyways, keeping the WRITE_ONCE + READ_ONCE for this is fine.

> Unfortunately it's not feasible to hold the reader lock in
> ssam_request_sync_submit() due to reentrancy. Specifically, as the lock,
> if at all (i.e. if this is not a client driver bound to the controller),
> must be held not only during submission but until the request has been
> completed. Note that if we would hold the lock during submission, this
> is just a smoke-test.

Ack.

<more snip>

Regards,

Hans