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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