linux-kernel - Re: [PATCH v1 3/4] drm/ls2kbmc: Add support for Loongson-2K BMC display

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Message-ID: <392f855c-4474-4e12-8bdc-3baf43d34e13@suse.de>
Date: Mon, 6 Jan 2025 15:10:24 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Binbin Zhou <zhoubb.aaron@...il.com>
Cc: Binbin Zhou <zhoubinbin@...ngson.cn>, Huacai Chen
 <chenhuacai@...ngson.cn>, Lee Jones <lee@...nel.org>,
 Corey Minyard <minyard@....org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Huacai Chen <chenhuacai@...nel.org>,
 linux-kernel@...r.kernel.org, openipmi-developer@...ts.sourceforge.net,
 dri-devel@...ts.freedesktop.org, Xuerui Wang <kernel@...0n.name>,
 loongarch@...ts.linux.dev, Chong Qiao <qiaochong@...ngson.cn>
Subject: Re: [PATCH v1 3/4] drm/ls2kbmc: Add support for Loongson-2K BMC
 display

Hi,

Thanks for the info.


Am 06.01.25 um 08:03 schrieb Binbin Zhou:
[...]
>> Could you point to the exact call that fails within simpledrm?
> If we use simpledrm directly, the following error occurs:
>
> [    8.289823] simple-framebuffer simple-framebuffer.0: [drm] *ERROR*
> could not acquire memory range [mem 0xe0031200000-0xe00315fffff flags
> 0x200]: -16
> [    8.312681] simple-framebuffer simple-framebuffer.0: probe with
> driver simple-framebuffer failed with error -16
>
> The reason for the failure: overlapping resources.
>
> https://elixir.bootlin.com/linux/v6.12.6/source/drivers/video/aperture.c#L175

This error means that there's already an instance of simpledrm bound to 
the BMC framebuffer. So you already have a working display and some 
graphics under Linux without the new driver, right?

If so, why do you need a new driver that does exactly the same as simpledrm?

Best regards
Thomas

>>> Because although we register the driver in platform form, its memory
>>> belongs to pci space and we can see the corresponding pci probe and
>>> resource allocation in Patch-1.
>> I don't understand. Graphics memory is often located on the PCI bus.
>> What is so special about this one?
>>
>>> Therefore, we need to use aperture_remove_conflicting_pci_devices().
>> So there is already a device that represents the graphics card? That's
>> what you'd remove here? If you only add that MFD device, who owns the
>> framebuffer? If it's the PCI device from patch 1 ("ls2k-bmc"), why does
>> aperture_remove_conflicting_pci_devices() not remove that device? I'm
>> somewhat confused, because the logic in your driver mostly looks like it
>> binds to a pre-configured framebuffer, but some of the code doesn't.
> Perhaps the use of aperture_remove_conflicting_pci_devices() is wrong,
> as there is only one display device for the LS2K BMC and there will be
> no phase conflict.
>
> When I tried to use that API before, it was partly due to the error
> above, and partly because I referenced that other display drivers via
> pci_driver.probe() would have it, just in case I used it, which was
> probably the wrong choice.
>
> The resources for pci bar0 are as follows:
> BAR0: e0030000000/SZ_32M
>
> 0x0              0x600000  0xf00001c    16M            32M
> +----+--------------+--------+-----------+---+-----------------+
> | 2M | simpldrm |           | IPMI      |     | video env     |
> +-----------------------------------------------------------------+
>
> The mfd driver registers the ls2kbmc-framebuffer and ls2k-ipmi-si
> devices according to the resource allocation shown above. At the same
> time, the ls2kbmc drm is bound to the pre-configured “simpldrm”
> resource in the above figure, which is passed through the
> ls2kbmc-framebuffer driver. In addition, the resolution is read from
> “video env” for the time being, and the resolution adaption is planned
> to be added later.
>
>> Best regards Thomas
>>
>>> Also, since we are using BMC display, the display will be disconnected
>>> when BMC reset, at this time we need to push the display data (crtc,
>>> connector, etc.) manually as shown in Patch-4.
>>>
>>> Probably it's not the most suitable way to implement it.
>>>
>>>> Best regards
>>>> Thomas
>>>>
>>>>> +     },
>>>>> +     .probe = ls2kbmc_probe,
>>>>> +     .remove = ls2kbmc_remove,
>>>>> +};
>>>>> +
>>>>> +module_platform_driver(ls2kbmc_platform_driver);
>>>>> +
>>>>> +MODULE_DESCRIPTION("DRM driver for Loongson-2K BMC");
>>>>> +MODULE_LICENSE("GPL");
>>>> --
>>>> --
>>>> Thomas Zimmermann
>>>> Graphics Driver Developer
>>>> SUSE Software Solutions Germany GmbH
>>>> Frankenstrasse 146, 90461 Nuernberg, Germany
>>>> GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
>>>> HRB 36809 (AG Nuernberg)
>>>>
>> --
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Frankenstrasse 146, 90461 Nuernberg, Germany
>> GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
>> HRB 36809 (AG Nuernberg)
>>
>
> --
> Thanks.
> Binbin

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)