linux-kernel - Re: [PATCH v2 1/2] drm: Implement DRM aperture helpers under video/

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Date:   Tue, 21 Jun 2022 14:25:24 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Alex Williamson <alex.williamson@...hat.com>, corbet@....net,
        maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        airlied@...ux.ie, daniel@...ll.ch, deller@....de,
        gregkh@...uxfoundation.org
Cc:     dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 1/2] drm: Implement DRM aperture helpers under video/

Hello Thomas,

On 6/21/22 13:29, Thomas Zimmermann wrote:

[...]

>>> +
>>> +static bool overlap(resource_size_t base1, resource_size_t end1,
>>> +		    resource_size_t base2, resource_size_t end2)
>>> +{
>>> +	return (base1 < end2) && (end1 > base2);
>>> +}
>>
>> There's a resource_overlaps() helper in include/linux/ioport.h, I wonder if it
>> could just be used, maybe declaring and filling a struct resource just to call
>> that helper. Later as an optimization a resource_range_overlap() or something
>> could be proposed for include/linux/ioport.h.
> 
> Bu then we'd have to declare struct resource-es for using an interface. 
> This helper is trivial. If anything, resource_overlaps() should be 
> generalized.
>

Yes, that works too. Probably then we should just keep as is and then as a follow
up we can add another helper to include/linux/ioport.h to avoid having something
that's only for the aperture helpers.

>>
>> Also, I noticed that resource_overlaps() uses <= and >= but this helper uses
>> < and >. It seems there's an off-by-one error here but maybe I'm wrong on this.
> 
> struct resource stores the final byte of the resource. In our case 'end' 
> is the byte after that. So the code is correct.
> 
> Do we ever have resources that end at the top-most byte of the address 
> space?
>

I don't know to be honest.

[...]

>>> +static void detach_platform_device(struct device *dev)
>>> +{
>>> +	struct platform_device *pdev = to_platform_device(dev);
>>> +
>>> +	/*
>>> +	 * Remove the device from the device hierarchy. This is the right thing
>>> +	 * to do for firmware-based DRM drivers, such as EFI, VESA or VGA. After
>>> +	 * the new driver takes over the hardware, the firmware device's state
>>> +	 * will be lost.
>>> +	 *
>>> +	 * For non-platform devices, a new callback would be required.
>>> +	 *
>>
>> I wonder if we ever are going to need this. AFAICT the problem only happens for
>> platform devices. Or do you envision a case when some a bus could need this and
>> the aperture unregister the device instead of the Linux kernel device model ?
>>
> 
> In the current code, we clearly distinguish between the device and the 
> platform device. The latter is only used in a few places where it's 
> absolutely necessary, because there's no generic equivalent to 
> platform_device_unregister(). (device_unregister() is something else 
> AFAICT.)  At some point, I'd like to see the aperture code being handled 
> in a more prominent place within resource management. That would need it 
> to use struct device.
>

Ok, I was wondering what was the value of the indirection level other than
making the code more complex and supporting an hypothetical case of a FW
driver that would not bind against a platform device.

But if the goal is to move this at some point to a more generic place (i.e:
the Linux device model itself) then I agree that we can just keep it as is.

When you re-spin this patch, feel free to add my R-b since looks good to me.

And thanks again for working on this!

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat