[PATCH 9/9] kernel/x86: remove DRM support

Philip Prindeville philipp_subx at redfish-solutions.com
Mon May 1 08:09:42 PDT 2023

> On Apr 28, 2023, at 9:45 PM, Elliott Mitchell <ehem+openwrt at m5p.com> wrote:
> On Thu, Apr 27, 2023 at 11:21:28AM +0200, Thibaut wrote:
>>> Le 27 avr. 2023 à 02:11, Elliott Mitchell <ehem+openwrt at m5p.com> a écrit :
>>> On Thu, Apr 27, 2023 at 12:50:52AM +0200, Stefan Lippers-Hollmann wrote:
>>>> On 2023-04-19, Elliott Mitchell wrote:
>>>>> Direct Rendering Manager is mainly for running X (possibly Wayland
>>>>> too).  As OpenWRT is meant for networking devices, there is no need
>>>>> for the support to be present.
>>>> That is only partially true, the Linux kernel is making a strong push
>>>> away from deprecated (FB_*) graphics drivers to DRM based ones, with 
>>>> kernel based mode setting this is getting more (any) attention for 
>>>> console support as well. Even without getting anywhere near X/ Wayland,
>>>> there is more than just a 80x25 tty on real hardware (and even VMs).
>>> Real x86 hardware often has the capability to use a serial port as
>>> console.  The conventional UEFI implementation fully supports this use
>>> case.  I can well believe a number of manufacturers disabling the
>>> functionality though.
>>> VMs *can* have more than a 80x25 tty.  By the time you're getting to 4
>>> or more VMs you should be thinking about disabling the functionality due
>>> to the heavy overhead (unless the OS in the VM doesn't support serial
>>> consoles).
>> You seem to assume that x86 is only/mainly run on VMs.
>> That is not necessarily the case, and I see no reason to degrade device support that way.
> Okay, as already stated there are at least two solutions to this.
> 1> Turn most functionality into modules and include support for runtime
> loading of kernel modules.
> 2> Create more kernel variants for OpenWRT/x86.
>> Would you mind documenting the measurable gains from your changes, so we have some metric to assess their relevance?
> I had suspected as much, but fully disabling ISA DMA didn't directly
> have much impact (less than 4195 byte reduction).  I was already guessing
> most of the gain was CONFIG_ISA=n, but hoped purging ISA DMA might
> squeeze out a bit more.
> Removing AGP shrunk vmlinux.bin by 4KB.  Since the kernel is a multiple
> of page size, this means a reduction of 1-8191 bytes.  This though may
> have translated into a larger impact when CONFIG_DRM was set to no.
> Setting CONFIG_DRM=n resulted in a vmlinux.bin delta of -2203648 bytes.
> Not quite a 10% reduction in kernel runtime size, but close.  Here we
> have a major impact on kernel size.
> Removing USB support is certainly inappropriate for a desktop build, but
> appropriate for something using a serial console (some types of VM).
> That has a delta of -2117632 bytes.

I occasionally use the USB pass-thru with memory sticks to install or recover an VM.


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