On Thu, 7 Sep 2000, Justin Schoeman wrote: > Just a quick comment on this thread: (from the bt848 pdf) > "WRITE, WRITEC and SKIP ... control the sequence of packed data written > to target memory on a byte resolution basis." > > So there are no problems doing byte-aligned clips. I have tested this > myself (with bttv2), and it works perfectly (single pixel wide clips, > that is, but you obviously only get a small piece of the image before > running out of RISCMEM). I seem to remember looking at bttv's clipping > code once, and (at least at that stage), there were a number of possible > RISC memory overflows. Did you test it in 32bpp mode? With dword wide data, getting single pixel clips is easy. The overlay code in the v4l driver is pretty bad. It doesn't use WRITEC instructions, which means that each pixel would written with a WRITE instruction, which requires a destination address. While the bt848 documentation doesn't say one way or the other if an address must be dword aligned, I've seen a couple posts that say that they must. Just what the bt848 does with non-dword skip and write instructions is entirely unclear to me. The manual doesn't help much, but some bits that appear relevant: p43, "The FIFO block ... outputs DWORDs to the DMA Controller ..." Seems pretty clear that the FIFO can only pass data to the DMA controller in dword wide blocks. p48, "The Bt848's FIFO DWORDs are perfectly aligned to the PCI bus ..." So writing the dwords of data from the FIFO to the PCI bus to a non-aligned address isn't possible. p52, [SKIP instruction] "Skip ... count # of bytes from the FIFO. This may start and stop in the middle of a DWORD." Seems pretty clear that skipping non-dword aligned or long data is ok. This text does NOT appear in the descriptions of the WRITE* instructions, which implies that they are different. It's clear that writing byte wide data can be done using only dword wide writes and skips, with careful use of the byte enables. If the bt848 is smart enough do this automatically, and how it would do so, it not clear to me.
