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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Sep 23, 2024 at 12:12:39PM +0300, Jani Nikula wrote:
> 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";

Or could just be something like:

static const char vbt_signature[] = "$VBT";
static const int vbt_signature_len = 4;

BUILD_BUG_ON(vbt_signature_len != sizeof(vbt_signature) - 1);
BUILD_BUG_ON(vbt_signature_len != sizeof(u32));

and use vbt_signature_len instead sizeof(vbt_signature) everwhere.

> 
> >
> >> 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

-- 
Ville Syrjälä
Intel