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HID: introduce proper zeroing of unused bits in output reports 

Some HID devices are looking on the unused bits in the HID reports they
receive. This is violating the specification, but we want to make those
devices work. Well-behaving devices are unaffected, as they don't care
about the unused bits.

If bitsused % 8 is 0 all bits in data[] get used and we don't need to
clear anything. Otherwise (bitsused % 8) bits of the last byte get used.
By shifting 1 for (bitsused % 8) bits and subtracting 1 we create a mask
consisting of (bitsused % 8) ones and remaining zeroes. By ANDing we
clear the upper unused bits.

Signed-off-by: Simon Budig <simon@budig.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
This commit is contained in:
Simon Budig 2007-03-12 13:52:04 +01:00 committed by Jiri Kosina
parent a21bd69e15
commit 46386b5870
1 changed files with 5 additions and 0 deletions
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5
drivers/hid/hid-core.c
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@ -872,8 +872,13 @@ static void hid_output_field(struct hid_field *field, __u8 *data)
unsigned count = field->report_count;
unsigned offset = field->report_offset;
unsigned size = field->report_size;
unsigned bitsused = offset + count * size;
unsigned n;
/* make sure the unused bits in the last byte are zeros */
if (count > 0 && size > 0 && (bitsused % 8) != 0)
data[(bitsused-1)/8] &= (1 << (bitsused % 8)) - 1;
for (n = 0; n < count; n++) {
if (field->logical_minimum < 0) /* signed values */
implement(data, offset + n * size, size, s32ton(field->value[n], size));