Hi, On Sun, May 12, 2019 at 06:08:19PM +0200, U.Mutlu wrote: > Hi Maxime & Others, > > what follows is a somewhat lengthy technical story behind this patch; > you can just skip it and jump to the end. > > > As can be seen in the ahci_sunxi.c, the port used in this patch > is this one (32bit): > #define AHCI_P0DMACR 0x0170 > It's a so called "Vendor Specific Port" according to the SATA/AHCI specs by Intel. > The data behind it is actually a struct, consisting of 4 fields, > each 4bits long, plus a 16bits long field that is marked as Reserved > in secondary literature (see below): > > struct AHCI_P0DMACR_t > { > unsigned TXTS : 4, > RXTS : 4, > TXABL : 4, > RXABL : 4, > Res1 : 16; > }; > > This struct is just my creation for my own tests as it's not part of the > driver source. The patch touches only the first 2 fields: TXTS and RXTS. > > See this similar product documentation by Texas Instruments for the above struct: > https://www.ti.com/lit/ug/sprugj8c/sprugj8c.pdf > TMS320C674x/OMAP-L1x Processor, Serial ATA (SATA) Controller, User's Guide, > Literature Number: SPRUGJ8C, March 2011, > Page 68, Chapter 4.33 "Port DMA Control Register (P0DMACR)" > > The above TI document describes the two fields as follows: > > TXTS: > Transmit Transaction Size (TX_TRANSACTION_SIZE). This field defines the > DMA transaction size in > DWORDs for transmit (system bus read, device write) operation. [...] > > RXTS: > Receive Transaction Size (RX_TRANSACTION_SIZE). This field defines the > Port DMA transaction size > in DWORDs for receive (system bus write, device read) operation. [...] > > > So, in my patch the fields TXTS and RXTS are set to 3 each. > Without the patch, these fields seem to have some random content > (I'vee seen 5 and 6, 4 and 4, and 0 and 0 on different devices), > as the previous code doesn't touch these 2 fields (ie. these two fields > are not within the used old mask of 0xff00; cf. ahci_sunxi.c, function > ahci_sunxi_start_engine(...)). > > > Some background story in my hunt for obtaining product documentation: > > I couldn't find any product documentation for the SATA/AHCI > in these SoCs by Allwinner Technology (allwinnertech.com), > unlike with such products from other such companies. > > I asked Allwinner, but they just said that the A20 of my SBC > would (allegedly) no more be actual and that the support for it > has ended (but this statement somehow cannot be true as the > A20 SoC is still continued being marketed at their website). > They instead sent me a bunch of really irrelevant PDFs which have > nothing to do with SATA/AHCI. > > So, the company Allwinner Technology unfortunately was not cooperative > to provide me information on their SATA/AHCI-implementation in their SoCs :-( > Even the ports used in the actual ahci_sunxi.c in the linux tree are undocumented; > it is even commented with "/* This magic is from the original code */" > and below it many ports are used for which no documentation is available, > or at least I couldn't find any on the Internet. And the initial programmer > in 2014 and prior was Daniel Wang (danielwang@xxxxxxxxxxxxxxxxx), > but email to that address bounces. > > So, I was forced to research secondary literature from other vendors > like Texas Instruments (thanks TI !) and Intel, and also studying > very old source codes in the old Linux repositories (as it differs > much from the current version) going back to the year 2014, and had > to do many (blind) experiments until I found this solution. > > The above given User's Guide by Texas Instruments (and their such > documents for their newer such products) helped me much to find the solution. > It's of course not really the correct documentation for the Allwinner SoCs, > but still better than nothing. > > If I only had the right documentation, then I for sure could try > to further improve that already achieved result by this patch, > as with SATA-II upto 300 MiB/s is possible. > > > Yes, I'll resend the patch with some appropriate comments. That's awesome research and explanation, thanks! :) Maxime -- Maxime Ripard, Bootlin Embedded Linux and Kernel engineering https://bootlin.com