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Word alignment error in Linux PSP with EDMA crossbar code

Stan Hu
Prodigy 130 points
Other Parts Discussed in Thread: WL1271

When using MMC2 with DMA, I got the following kernel error:

[   60.190765] Alignment trap: not handling instruction f8531022 at [<c0015af8>]
[   60.198242] Unhandled fault: alignment exception (0x001) at 0xf9e10f9d
[   60.205139] Internal error: : 1 [#1]
[   60.208862] Modules linked in: arc4 wl12xx_sdio wl12xx mac80211 cfg80211 rfkill spidev ip_tables x_tables ipv6
[   60.219390] CPU: 0    Not tainted  (3.2.23 #196)
[   60.224273] PC is at map_xbar_event_to_channel+0x4c/0x68
[   60.229827] LR is at map_xbar_event_to_channel+0x39/0x68

This appears to be a problem with the implementation of map_xbar_event_to_channel in:

http://arago-project.org/git/projects/?p=linux-am33x.git;a=commit;f=arch/arm/mach-omap2/devices.c;hb=a7b46224febb00555eba44f0a162d3f774f1eab3

int map_xbar_event_to_channel(unsigned event, unsigned *channel,
798                         struct event_to_channel_map *xbar_event_mapping)
799 {
800         unsigned ctrl = 0;
801         unsigned xbar_evt_no = 0;
802         unsigned val = 0;
803         unsigned offset = 0;
804         unsigned mask = 0;
805
806         ctrl = EDMA_CTLR(event);
807         xbar_evt_no = event - (edma_info[ctrl]->num_channels);
808
809         if (event < edma_info[ctrl]->num_channels) {
810                 *channel = event;
811         } else if (event < edma_info[ctrl]->num_events) {
812                 *channel = xbar_event_mapping[xbar_evt_no].channel_no;
813                 /* confirm the range */
814                 if (*channel < EDMA_MAX_DMACH)
815                         clear_bit(*channel, edma_info[ctrl]->edma_unused);
816                 mask = (*channel)%4;
817                 offset = (*channel)/4;
818                 offset *= 4;
819                 offset += mask;
820                 val = (unsigned)__raw_readl(AM33XX_CTRL_REGADDR(
821                                         AM33XX_SCM_BASE_EDMA + offset));
822                 val = val & (~(0xFF));
823                 val = val | (xbar_event_mapping[xbar_evt_no].xbar_event_no);
824                 __raw_writel(val,
825                         AM33XX_CTRL_REGADDR(AM33XX_SCM_BASE_EDMA + offset));
826                 return 0;
827         } else {
828                 return -EINVAL;
829         }
830
831         return 0;
832 }
If the mapped channel is odd, then the offset passed to __raw_readl and __raw_writel on lines 820 and 825 will not be on a word-aligned boundary.
I replaced lines 816-825 with this to write the EDMA registers on word boundaries, and this problem went away:

offset = (*channel)/4;

offset *= 4;
val = (unsigned int)__raw_readl(AM33XX_CTRL_REGADDR(
AM33XX_SCM_BASE_EDMA + offset));

mask = ((*channel)%4);
mask *= 8;
val = val & (~((0xFF) << mask));
val = val | (xbar_event_mapping[xbar_evt_no].xbar_event_no << mask);
__raw_writel(val, AM33XX_CTRL_REGADDR(AM33XX_SCM_BASE_EDMA + offset));