On Thu, 2019-06-13 at 15:13 +0800, Ming Lei wrote: > Hi, > > Most of drivers use sg helpers to operate sgl, however there is > still a few drivers which operate sgl directly, this way can't > work in case of chained sgl. This isn't a useful explanation of the issue you make it sound like a bug, which it isn't: it's a change of behaviour we'd like to introduce in SCSI. Please reword the explanation more along the lines of --- Scsi MQ makes a large static allocation for the first scatter gather list chunk for the driver to use. This is a performance headache we'd like to fix by reducing the size of the allocation to a 2 element array. Doing this will break the current guarantee that any driver using SG_ALL doesn't need to use the scatterlist iterators and can get away with directly dereferencing the array. Thus we need to update all drivers to use the scatterlist iterators and remove direct indexing of the scatterlist array before reducing the initial scatterlist allocation size in SCSI. --- Which explains what we're trying to do and why. In particular changelogs like this > The current way isn't safe for chained sgl, so use sgl helper to > operate sgl. Are just plain wrong: They were perfectly safe until you altered the conditions for using non-chained sgls. Please use the above explanation in the patches, abbreviated if you like, so all recipients know why this needs doing and that it isn't an existing bug. James
