Re: [patches] [PATCH] [21/22] x86_64: Extend bzImage protocol for relocatable bzImage

[Eric W. Biederman]

"H. Peter Anvin" <hpa@xxxxxxxxx> writes:

> Eric W. Biederman wrote:
>> 
>> I expect I can find a few more examples where we specify
>> video_cols and video_lines but we use video_mode == 0.
>> 
>> Going farther mode 0x00 is a BIOS 40x25 mode.  So the patch below is
>> not always safe even if we boot the bzImage.  It is just highly
>> unlikely anyone would start the kernel in 40x25 text mode. 
>> 
>
> Mode 0x00 is, at least theoretically, BIOS 40x25 *grayscale*; this mode
> (and mode 0x02 which is the same thing in 80x25) were as far as I know
> only ever used with composite monitors off CGA cards, i.e. functionally
> never.  Actual monochrome monitors used mode 0x07.

I agree.  We are not at all likely to see it in practice.  Even
if my memory is correct and vga cards and non-monochrome cga
cards supported that mode.

That doesn't mean checking for 0x00 is sufficient to detect
an initialized struct screen_info, or a lack of a video screen.

We have in kernel historical precedent for using 0x00 as just meaning
a text mode.  I'm fairly certain that I looked I more closely I could
find this convention of using 0x00 to mean a text mode on ia64, mips,
and ppc, in addition to the instances I found on alpha, arm,

Since the whole point is to detect the case where we don't have
a screen at all it makes sense to check several additional variables
and make certain that they are all 0.  Agreed?

Eric
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