Date: Sun, 9 Nov 1997 08:40:52 -0500 From: Randall Hopper <rhh@ct.picker.com> To: Amancio Hasty <hasty@rah.star-gate.com> Cc: multimedia@freebsd.org Subject: bt848 bandwidth limitation Message-ID: <19971109084052.07049@ct.picker.com>
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Speaking of bandwidth limitations, I noticed this slightly puzzling
check in the driver to determine when to split a scanline transfer into
halves in the RISC program:
/* When to split a dma transfer , the bt848 has timing as well as
dma transfer size limitations so that we have to split dma
transfers into two dma requests
*/
#define DMA_BT848_SPLIT 319*2
...
if ((width * pixel_width) < DMA_BT848_SPLIT ) {
To take an example, if we're doing 4Bpp 640x480, a scanline will be 2560
bytes. This "( 640*4 < 638 )" check will of course trigger a scanline
split, but each half of the scanline is 1280 bytes -- over twice the value
of DMA_BT848_SPLIT.
So what's got me wondering is that this is such a small number and we only
ever split into 2 segments. Thoughts?
Randall
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