V4L/DVB (3766): Correct buffer size calculations in cx88-core.c

The computation in cx88_risc_buffer suffers from the mistake:
a non-zero padding value can cause more page borders to be crossed,
leading to big buffer over-runs.  
This patch changes the additive constant from 3 + 4 to 4
It also changees the constant in cx88_risc_databuffer from 3 + 4 to 2,
because 2 dwords are the correct vaule.

Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

1 files changed, 10 insertions, 6 deletions

diff --git a/drivers/media/video/cx88/cx88-core.c b/drivers/media/video/cx88/cx88-core.c
index 2c3d9f1999be..e1092d5d4628 100644
--- a/drivers/media/video/cx88/cx88-core.c
+++ b/drivers/media/video/cx88/cx88-core.c
@@ -146,9 +146,11 @@ int cx88_risc_buffer(struct pci_dev *pci, struct btcx_riscmem *risc,
                 fields++;
 
         /* estimate risc mem: worst case is one write per page border +
-           one write per scan line + syncs + jump (all 2 dwords) */
-        instructions  = (bpl * lines * fields) / PAGE_SIZE + lines * fields;
-        instructions += 3 + 4;
+           one write per scan line + syncs + jump (all 2 dwords).  Padding
+           can cause next bpl to start close to a page border.  First DMA
+           region may be smaller than PAGE_SIZE */
+        instructions  = fields * (1 + ((bpl + padding) * lines) / PAGE_SIZE + lines);
+        instructions += 2;
         if ((rc = btcx_riscmem_alloc(pci,risc,instructions*8)) < 0)
                 return rc;
 
@@ -176,9 +178,11 @@ int cx88_risc_databuffer(struct pci_dev *pci, struct btcx_riscmem *risc,
         int rc;
 
         /* estimate risc mem: worst case is one write per page border +
-           one write per scan line + syncs + jump (all 2 dwords) */
-        instructions  = (bpl * lines) / PAGE_SIZE + lines;
-        instructions += 3 + 4;
+           one write per scan line + syncs + jump (all 2 dwords).  Here
+           there is no padding and no sync.  First DMA region may be smaller
+           than PAGE_SIZE */
+        instructions  = 1 + (bpl * lines) / PAGE_SIZE + lines;
+        instructions += 1;
         if ((rc = btcx_riscmem_alloc(pci,risc,instructions*8)) < 0)
                 return rc;