On Wed, Nov 09, 2022 at 02:36:11PM +0000, Daniel Golle wrote: > On Wed, Nov 09, 2022 at 01:58:29PM +0000, Matthew Wilcox wrote: > > ... actually, why can't you call read_part_sector() and avoid all of > > this? > > I've tried that before and the problem is that read_part_sector() > returns a pointer to one sector (typically 512 bytes) of data. > And this pointer should not be accesses beyond sector boundaries, > right? You'd have to call read_part_sector() again for the next > sector. > > The FIT structure, however, usually exceeds the size of one sector, > and having a continous memory area covering the structure as a whole > is crucial for libfdt to do its job. > > I could, of course, use read_part_sector() to copy all sectors > covering the FIT structure into a buffer, but that seemed strange > given that read_part_sector() actually used read_mapping_page() > (and now uses read_mapping_folio()) internally and then returns a > pointer to the offset within the page/folio. So why not read it in one > piece in first place instead of having it first split up to sectors > by read_part_sector() just to then having to reassemble it into a > continous buffer again. Are you guaranteed that it's "sufficiently" aligned on storage so that it fits entirely within a single page? If not, you'll have to copy it, vmap it, or fix libfdt to handle a segmented buffer.
