On Sun, Nov 25, 2018 at 11:11:05AM +0000, Russell King - ARM Linux wrote: > On Sat, Nov 24, 2018 at 05:07:17PM -0800, Tony Lindgren wrote: > > * Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> [181124 20:10]: > > > On Fri, Nov 23, 2018 at 08:52:15PM +0200, Aaro Koskinen wrote: > > > > Hi, > > > > > > > > On Fri, Nov 23, 2018 at 02:35:04PM +0200, Peter Ujfalusi wrote: > > > > > On 22/11/2018 17.12, Russell King - ARM Linux wrote: > > > > > > I'm also not sure about this: > > > > > > > > > > > > if (cpu_is_omap15xx()) > > > > > > end++; > > > > > > > > > > > > in dma_dest_len() - is that missing from the omap-dma driver? It looks > > > > > > like a work-around for some problem on OMAP15xx, but I can't make sense > > > > > > about why it's in the UDC driver rather than the legacy DMA driver. > > > > > > > > > > afaik no other legacy drivers were doing similar thing, this must be > > > > > something which is needed for the omap_udc driver to fix up something? > > > > > > > > Here's the patch that added it: https://marc.info/?l=linux-omap&m=119634396324221&w=2 > > > > > > > > "Make DMA-OUT behave on the 1510 ... the 1510 CPC register was just > > > > off-by-one with respect to the 1611 CDAC" > > > > > > ... which suggests that's a problem with the CPC register itself, and > > > we should fix that in the DMAengine driver rather than the USB gadget > > > driver. > > > > > > Tony, any input on this? > > > > Yeah that sounds like some hardware work-around for 15xx as described > > in the DMA_DEST_LAST macro reading CSAC on 15xx instead of CDAC. Seems > > like it should be done in the dmaengine driver.. My guess is that other > > dma users never needed to read CSAC register? > > Hmm, reading the OMAP1510 TRM for the CPC register, it seems that > omap-dma won't handle this particularly well. The fact that the > register only updates after the last request in an element or frame > means that if we try to use this value as the current source / > destination address before the first transfer, it can be wildly > wrong. > > Saving the current value at the beginning of a request, and detecting > if it's changed (like omap_udc) isn't going to work well in the > generic case. If, say, the register happens to contain 0x0004, and > our next transfer is using 32-bit elements in element sync mode > starting at address with lsb 16 bits as 0, it would mean we'd see > 0x0004 in this register after the _second_ element has been > transferred, and we'd assume that the register hasn't yet changed - > but we would have in reality transferred two elements. > > However, omap-dma.c zeros the CPC register before each transfer, > which is interesting, because in one place the OMAP1510 TRM says > that the CPC register is read/write, but in the actual description > of this register, it says it's read-only. > > What it also means is that, in such a case, after the 2nd element has > been transferred, where the register contains 0x0004, the address > we'd be looking for (to calculate the residual) should be 0x0008, > not 0x0005, so we actually need to be adding the number of bytes > according to element size. > > Looking at omap-dma.c, someone has added support for the residue > granularity: > > if (__dma_omap15xx(od->plat->dma_attr)) > od->ddev.residue_granularity = > DMA_RESIDUE_GRANULARITY_DESCRIPTOR; > else > od->ddev.residue_granularity = DMA_RESIDUE_GRANULARITY_BURST; I'll point out that this was introduced by: commit c9bd0946da243a8eb86b44ff613e2c813f9b683b Author: Janusz Krzysztofik <jmkrzyszt@xxxxxxxxx> Date: Tue Jun 5 18:59:57 2018 +0200 dmaengine: ti: omap-dma: Fix OMAP1510 incorrect residue_granularity ... It was verified already before that omap_get_dma_src_pos() from arch/arm/plat-omap/dma.c didn't work correctly for OMAP1510 - see commit 1bdd7419910c ("ASoC: OMAP: fix OMAP1510 broken PCM pointer callback") for details. Apparently the same applies to its successor, omap_dma_get_src_pos() from drivers/dma/ti/omap-dma.c. So, this now presents us with bigger problems - if we fix it now for omap_udc, should the above commit be reverted, and if we do revert the above commit, will it regress OMAP1510 audio. The intention of omap-dma was always to report an accurate residue. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
