On Saturday, October 17, 2015 at 05:22:41 AM, Thomas Chou wrote: > Hi Marek,
Hi! > On 10/17/2015 07:03 AM, Marek Vasut wrote: > >> I would suggest the "cache alignment check and skip" be removed from > >> cache flush ops, and say out the DMA buffer allocation rule loudly in > >> README, and enforce it by guardianship. > > > > What exactly do you envision by this "guardianship" ? > > I mean the reviews of custodians. Wouldn't an automated check be better ? It's easier and costs almost nothing. Besides, custodians are not perfect and cannot detect all the issues. > >> Please allow me to restate the reasons, > >> > >> 1. The cache flush ops are commonly used. Please refer to the "Cache and > >> TLB Flushing Under Linux" doc, linux/Documentation/cachetlb.txt. > >> Violating the defined interface is much worse than violating coding > >> style. It will certainly impact the portability of u-boot. And might > >> introduce more bug than resolve. > > > > I agree with this one. > > > >> 2. We all agree that enforcing DMA buffer allocation to cache aligned is > >> the only real solution. Adding such "check and skip" to cache flush ops > >> cannot prevent the flush or solve the problem. > > > > We should probably check-scream-skip here. > > > >> 3. Though the flush size of block device are usually aligned, the size > >> of packet are not. Asking the packet drivers to adjust the flush size > >> does not make sense. It is the job of cache flush ops. The debug probe > >> should not override the original purpose. It should be spelled for > >> common understanding. > > > > The socket buffer(s) should be aligned, so network packets should be > > fine. > > While the start of socket buffer might be aligned, the size of the > transfer might not for the send ops. It is depended on the net/tcp/ip > packets size. Aurgh :-( Now I see what you mean. This is purely bad, very bad. Here is a real possibility for corruption of variables close to the allocated DMA buffer, right ? > For example, with tftp, there is a lot of unaligned end of packets. > > tftp d1000000 u-boot-dtb.bin > > flush unaligned d7ff7020-d7ff704e > [repeat ..] > > So, such an alarm may be false. And such a skip can be bug. > > In fact, for my own projects, I have changed the memory allocation to > always cache aligned. And I rarely worry about it ever after. > > I look at the net.c of u-boot. There are packets buffer allocated on BSS > and stack. I would suggest avoid such programming, and use aligned > memory allocation stead. The stack allocation there is used because it's slightly faster and you don't need mallocator for that. I guess this is a topic for a broader discussion and we should include Tom and others into it. Would you mind starting another thread on the ML and CCing me, Tom Rini, Simon Glass etc please ? Best regards, Marek Vasut _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
