Re: [PATCH 4/5] drm/i915/bios: Extract vbt_signature[]

[Jani Nikula <jani.nikula@linux.intel.com>]

On Fri, 20 Sep 2024, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> On Thu, Sep 12, 2024 at 03:15:52PM +0300, Jani Nikula wrote:
>> On Tue, 10 Sep 2024, Ville Syrjala <ville.syrjala@linux.intel.com> wrote:
>> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> >
>> > Replace the three hand rolled "$VBT"s with a vbt_signature[]
>> > to avoid accidents.
>> >
>> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> > ---
>> >  drivers/gpu/drm/i915/display/intel_bios.c | 12 +++++++++---
>> >  1 file changed, 9 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/i915/display/intel_bios.c b/drivers/gpu/drm/i915/display/intel_bios.c
>> > index cbbda94c3dab..0b92b494117f 100644
>> > --- a/drivers/gpu/drm/i915/display/intel_bios.c
>> > +++ b/drivers/gpu/drm/i915/display/intel_bios.c
>> > @@ -2964,6 +2964,8 @@ static const struct bdb_header *get_bdb_header(const struct vbt_header *vbt)
>> >  	return _vbt + vbt->bdb_offset;
>> >  }
>> >  
>> > +static const char vbt_signature[4] = "$VBT";
>> 
>> Always a bit scary to add strings without termination, but I guess it's
>> justified here.
>
> I guess we could make it look less like a string with eg.
> static const u8 vbt_signature[] = { '$', 'V', 'B', 'T' };
> ?

Alternatively,

static const char *vbt_signature = "$VBT";

>
>> Reviewed-by: Jani Nikula <jani.nikula@intel.com>
>> 
>> 
>> > +
>> >  /**
>> >   * intel_bios_is_valid_vbt - does the given buffer contain a valid VBT
>> >   * @display:	display device
>> > @@ -2986,7 +2988,7 @@ bool intel_bios_is_valid_vbt(struct intel_display *display,
>> >  		return false;
>> >  	}
>> >  
>> > -	if (memcmp(vbt->signature, "$VBT", 4)) {
>> > +	if (memcmp(vbt->signature, vbt_signature, sizeof(vbt_signature))) {

But then this would need strlen()

>> >  		drm_dbg_kms(display->drm, "VBT invalid signature\n");
>> >  		return false;
>> >  	}
>> > @@ -3082,9 +3084,11 @@ static struct vbt_header *spi_oprom_get_vbt(struct intel_display *display,
>> >  	oprom_offset = intel_uncore_read(&i915->uncore, OROM_OFFSET);
>> >  	oprom_offset &= OROM_OFFSET_MASK;
>> >  
>> > +	BUILD_BUG_ON(sizeof(vbt_signature) != sizeof(u32));

And maybe this could be sizeof(vbt_signature) < sizeof(u32)

All of the alternatives are a bit disappointing, so maybe let's just
roll with the patch you already have here?

BR,
Jani.

>> > +
>> >  	for (count = 0; count < oprom_size; count += 4) {
>> >  		data = intel_spi_read32(&i915->uncore, oprom_offset + count);
>> > -		if (data == *((const u32 *)"$VBT")) {
>> > +		if (data == *((const u32 *)vbt_signature)) {
>> >  			found = oprom_offset + count;
>> >  			break;
>> >  		}
>> > @@ -3144,9 +3148,11 @@ static struct vbt_header *oprom_get_vbt(struct intel_display *display,
>> >  	if (!oprom)
>> >  		return NULL;
>> >  
>> > +	BUILD_BUG_ON(sizeof(vbt_signature) != sizeof(u32));
>> > +
>> >  	/* Scour memory looking for the VBT signature. */
>> >  	for (i = 0; i + 4 < size; i += 4) {
>> > -		if (ioread32(oprom + i) != *((const u32 *)"$VBT"))
>> > +		if (ioread32(oprom + i) != *((const u32 *)vbt_signature))
>> >  			continue;
>> >  
>> >  		p = oprom + i;
>> 
>> -- 
>> Jani Nikula, Intel

-- 
Jani Nikula, Intel