Message-ID: <415eb089-4a45-96e9-1816-88ba9c9be9aa@codeaurora.org>
Date:   Wed, 22 Jul 2020 10:33:01 -0700
From:   Siddharth Gupta <sidgup@...eaurora.org>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     agross@...nel.org, ohad@...ery.com, corbet@....net,
        linux-remoteproc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, tsoni@...eaurora.org,
        psodagud@...eaurora.org, rishabhb@...eaurora.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 1/2] remoteproc: Add remoteproc character device
 interface


On 7/22/2020 10:18 AM, Mathieu Poirier wrote:
> On Tue, Jul 21, 2020 at 01:56:35PM -0700, Bjorn Andersson wrote:
>> On Tue 21 Jul 12:16 PDT 2020, Siddharth Gupta wrote:
>>> On 7/15/2020 2:51 PM, Mathieu Poirier wrote:
>>>> On Wed, Jul 15, 2020 at 02:18:39PM -0600, Mathieu Poirier wrote:
>>>>> On Tue, Jul 07, 2020 at 12:07:49PM -0700, Siddharth Gupta wrote:
>> [..]
>>>>>> diff --git a/drivers/remoteproc/remoteproc_cdev.c b/drivers/remoteproc/remoteproc_cdev.c
>> [..]
>>>>>> +int rproc_char_device_add(struct rproc *rproc)
>>>>>> +{
>>>>>> +	int ret;
>>>>>> +	dev_t cdevt;
>>>>>> +
>>>>>> +	cdev_init(&rproc->char_dev, &rproc_fops);
>>>>>> +	rproc->char_dev.owner = THIS_MODULE;
>>>>>> +
>>>>>> +	cdevt = MKDEV(rproc_major, rproc->index);
>>>>>> +	ret = cdev_add(&rproc->char_dev, cdevt, 1);
>>>> Trying this patchset on my side gave me the following splat[1].  After finding
>>>> the root case I can't understand how you haven't see it on your side when you
>>>> tested the feature.
>>>>
>>>> [1]. https://pastebin.com/aYTUUCdQ
>> Mathieu, I've looked at this back and forth. Afaict this implies that
>> rproc_major is still 0. Could it be that either alloc_chrdev_region()
>> failed or somehow has yet to be called when you hit this point?
> That is exacly what I thought when I first stumbled on this but instrumenting
> the code showed otherwise.
>
> After function rproc_init_cdev() has been called @rproc_major contains the
> dynamically allocated major number in the upper 12 bits and the base minor
> number in the lower 20 bits.
>
> In rproc_char_device_add() we find this line:
>
>          cdevt = MKDEV(rproc_major, rproc->index);
>
> Macro MKDEV() builds a device number by shifting @rproc_major by 20 bits to the
> left and OR'ing that with @rproc->index.  But the device's major number is
> already occupying the upper 12bits, so shifthing another 20 bits to the left
> makes the major portion of the device number '0'.  That is causing cdev_add() to
> complain bitterly.
>
> The right way to do this is:
>
>          cdevt = MKDEV(MAJOR(rproc_major), rproc->index);
>
> Once I found the problem I thought about 32/64 bit issues.  Since Siddharth is
> using a 64bit application processor shifting another 20 bits would still have
> yielded a non-zero value.  But that can't be since dev_t is a u32 in
> linux/types.h.
>
> As such I can't see how it is possible to not hit that problem on a 64bit
> platform.
Hey Mathieu,

I just checked my testing tree for our devices and realized that I have 
an older version
of the patch. Hence I was unable to reproduce the error. I will fix this 
problem, and
send out a new patchset today.

Sorry about this error!

Thanks,
Sid
>>> Hey Mathieu,
>>>
>>> We aren't able to reproduce the error that you are seeing, the splat is
>>> coming
>>> from the check for whiteout device[1] - which shouldn't happen because of
>>> the
>>> find_dynamic_major call[2], right?
>>>
>>> We are successfully seeing all our character device files and able to
>>> successfully boot remoteprocs. From what I read and understood about
>>> whiteout
>>> devices they will be hidden in the fs.
>>>
>>> Could you provide more details about your configuration and testing?
>>>
>>> [1]: https://github.com/torvalds/linux/blob/master/fs/char_dev.c#L486
>>> <https://github.com/torvalds/linux/blob/master/fs/char_dev.c#L123>
>>> [2]: https://github.com/torvalds/linux/blob/master/fs/char_dev.c#L123
>>>
>>> <https://github.com/torvalds/linux/blob/master/fs/char_dev.c#L486>
>>>>>> +	if (ret < 0)
>>>>>> +		goto out;
>>>>>> +
>>>>>> +	rproc->dev.devt = cdevt;
>>>>>> +out:
>>>>>> +	return ret;
>>>>>> +}
>>>>>> +
>>>>>> +void rproc_char_device_remove(struct rproc *rproc)
>>>>>> +{
>>>>>> +	__unregister_chrdev(rproc_major, rproc->index, 1, "remoteproc");
>>>>>> +}
>>>>>> +
>>>>>> +void __init rproc_init_cdev(void)
>>>>>> +{
>>>>>> +	int ret;
>>>>>> +
>>>>>> +	ret = alloc_chrdev_region(&rproc_major, 0, NUM_RPROC_DEVICES, "remoteproc");
>>>>>> +	if (ret < 0)
>>>>>> +		pr_err("Failed to alloc rproc_cdev region, err %d\n", ret);
>>>>>> +}
>>>>>> +
>>>>>> +void __exit rproc_exit_cdev(void)
>>>>>> +{
>>>>>> +	unregister_chrdev_region(MKDEV(rproc_major, 0), NUM_RPROC_DEVICES);
>>>>> Please go back to the comment I made on this during my last review and respin.
>>>> After digging in the code while debugging the above problem, I don't see how
>>>> unregistering the chrdev region the way it is done here would have worked.
>>> Since this is compiled statically and not built as a module, we will never
>>> exercise the code path, so I will remove it in the next patchset.
>>>
>> You're right Siddharth, since we changed CONFIG_REMOTEPROC to bool it's no longer
>> possible to hit remoteproc_exit(), so you can omit this function
>> entirely. (And we should clean up the rest of that as well)
>>
>> [..]
>>>>>> diff --git a/include/linux/remoteproc.h b/include/linux/remoteproc.h
>> [..]
>>>>>> @@ -488,6 +489,8 @@ struct rproc_dump_segment {
>>>>>>     * @auto_boot: flag to indicate if remote processor should be auto-started
>>>>>>     * @dump_segments: list of segments in the firmware
>>>>>>     * @nb_vdev: number of vdev currently handled by rproc
>>>>>> + * @char_dev: character device of the rproc
>>>>>> + * @cdev_put_on_release: flag to indicate if remoteproc should be shutdown on @char_dev release
>>>>>>     */
>>>>>>    struct rproc {
>>>>>>    	struct list_head node;
>>>>>> @@ -523,6 +526,8 @@ struct rproc {
>>>>>>    	int nb_vdev;
>>>>>>    	u8 elf_class;
>>>>>>    	u16 elf_machine;
>>>>>> +	struct cdev char_dev;
>> As stated privately, I assumed based on this name that this is a struct
>> device related to that character device. So please rename this cdev to
>> save me from doing this mistake again.
>>
>> Thanks,
>> Bjorn

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