linux-ext4 - Re: [PATCH v6 04/13] drm/amdkfd: add SPM support for SVM

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Date:   Mon, 16 Aug 2021 14:54:30 -0400
From:   Felix Kuehling <felix.kuehling@....com>
To:     Christoph Hellwig <hch@....de>, Alex Sierra <alex.sierra@....com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        rcampbell@...dia.com, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, jgg@...dia.com, jglisse@...hat.com
Subject: Re: [PATCH v6 04/13] drm/amdkfd: add SPM support for SVM

Am 2021-08-15 um 5:10 a.m. schrieb Christoph Hellwig:
>> @@ -880,17 +881,22 @@ int svm_migrate_init(struct amdgpu_device *adev)
>>  	 * should remove reserved size
>>  	 */
>>  	size = ALIGN(adev->gmc.real_vram_size, 2ULL << 20);
>> -	res = devm_request_free_mem_region(adev->dev, &iomem_resource, size);
>> +	if (xgmi_connected_to_cpu)
>> +		res = lookup_resource(&iomem_resource, adev->gmc.aper_base);
>> +	else
>> +		res = devm_request_free_mem_region(adev->dev, &iomem_resource, size);
>> +
> Can you explain what the point of the lookup_resource is here? res->start
> is obviously identical to the start value you pass in.  So this is used
> as a way to query the length, but I'm pretty sure the driver must
> already know that as it inserted the resource itself, right?

I think you're right. We only need the start and end address from
lookup_resource and we already know that anyway. It means we can drop
patch 3 from the series.

Just to be sure, we'll confirm that the end address determined by our
driver matches the one from lookup_resource (coming from the system
address map in the system BIOS). If there were a mismatch, it would
probably be a bug (in the driver or the BIOS) that we'd need to fix anyway.


>
> On a slightly higher level comment svm_migrate_init is a bit of a mess
> with all the if/else already, and with the above addressed will become
> a bit more.  I think splitting it into a device private and device
> generic case would probably help people finding it to understand the code
> much better later on.  Even more so with a useful comment.

I don't really see the "mess" you're talking about. Including the above,
there are only 3 conditional statements in that function that are not
error-handling related:

        /* Page migration works on Vega10 or newer */
        if (kfddev->device_info->asic_family < CHIP_VEGA10)
                return -EINVAL;
...
        if (xgmi_connected_to_cpu)
                res = lookup_resource(&iomem_resource, adev->gmc.aper_base);
        else
                res = devm_request_free_mem_region(adev->dev, &iomem_resource, size);
...
        pgmap->type = xgmi_connected_to_cpu ?
                                MEMORY_DEVICE_GENERIC : MEMORY_DEVICE_PRIVATE;


Regards,
  Felix