Re: [PATCH 3/5] drm/i915/bios: Extract intel_spi_read16()

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

On Thu, Sep 12, 2024 at 03:02:03PM +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>
> >
> > The SPI VBT codepath only knows how to read 4 bytes at a time.
> > So to read the 2 byte vbt_size it masks out the unwanted msbs.
> > Hide that little implementation detail inside a new intel_spi_read16()
> > helper. Alse rename the existing intel_spi_read() to intel_spi_read32()
> > to make it clear what it does.
> >
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_bios.c | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_bios.c b/drivers/gpu/drm/i915/display/intel_bios.c
> > index cc4a4cc2bf3e..cbbda94c3dab 100644
> > --- a/drivers/gpu/drm/i915/display/intel_bios.c
> > +++ b/drivers/gpu/drm/i915/display/intel_bios.c
> > @@ -3053,13 +3053,18 @@ static struct vbt_header *firmware_get_vbt(struct intel_display *display,
> >  	return vbt;
> >  }
> >  
> > -static u32 intel_spi_read(struct intel_uncore *uncore, u32 offset)
> > +static u32 intel_spi_read32(struct intel_uncore *uncore, u32 offset)
> >  {
> >  	intel_uncore_write(uncore, PRIMARY_SPI_ADDRESS, offset);
> >  
> >  	return intel_uncore_read(uncore, PRIMARY_SPI_TRIGGER);
> >  }
> >  
> > +static u16 intel_spi_read16(struct intel_uncore *uncore, u32 offset)
> > +{
> > +	return intel_spi_read32(uncore, offset) & 0xffff;
> > +}
> > +
> >  static struct vbt_header *spi_oprom_get_vbt(struct intel_display *display,
> >  					    size_t *size)
> >  {
> > @@ -3078,7 +3083,7 @@ static struct vbt_header *spi_oprom_get_vbt(struct intel_display *display,
> >  	oprom_offset &= OROM_OFFSET_MASK;
> >  
> >  	for (count = 0; count < oprom_size; count += 4) {
> > -		data = intel_spi_read(&i915->uncore, oprom_offset + count);
> > +		data = intel_spi_read32(&i915->uncore, oprom_offset + count);
> >  		if (data == *((const u32 *)"$VBT")) {
> >  			found = oprom_offset + count;
> >  			break;
> > @@ -3094,9 +3099,8 @@ static struct vbt_header *spi_oprom_get_vbt(struct intel_display *display,
> >  	}
> >  
> >  	/* Get VBT size and allocate space for the VBT */
> > -	vbt_size = intel_spi_read(&i915->uncore,
> > -				  found + offsetof(struct vbt_header, vbt_size));
> > -	vbt_size &= 0xffff;
> > +	vbt_size = intel_spi_read16(&i915->uncore,
> > +				    found + offsetof(struct vbt_header, vbt_size));
> 
> Pedantically if vbt_size was the last member of struct vbt_header this
> could read past the checked size, but it's not and meh. Also nothing to
> do with this change, apart from this hiding the detail. Still meh.

This code raises a lot of other lingering questions as well:
- do 8/16 bit accesses not work at all?
- what happens on an unaligned 32bit access?

> 
> Reviewed-by: Jani Nikula <jani.nikula@intel.com>
> 
> 
> >  
> >  	if (vbt_size > oprom_size - count) {
> >  		drm_dbg(display->drm,
> > @@ -3109,7 +3113,7 @@ static struct vbt_header *spi_oprom_get_vbt(struct intel_display *display,
> >  		goto err_not_found;
> >  
> >  	for (count = 0; count < vbt_size; count += 4)
> > -		*(vbt + store++) = intel_spi_read(&i915->uncore, found + count);
> > +		*(vbt + store++) = intel_spi_read32(&i915->uncore, found + count);
> >  
> >  	if (!intel_bios_is_valid_vbt(display, vbt, vbt_size))
> >  		goto err_free_vbt;
> 
> -- 
> Jani Nikula, Intel

-- 
Ville Syrjälä
Intel