From: Xu Yilun <yilun.xu@xxxxxxxxx> > Sent: 13 August 2020 10:04 > > On Thu, Aug 13, 2020 at 08:28:05AM +0000, David Laight wrote: > > From: Xu Yilun > > > Sent: 13 August 2020 08:59 > > > On Wed, Aug 12, 2020 at 08:52:39AM +0000, David Laight wrote: > > > > From: Moritz Fischer > > > > > Sent: 12 August 2020 04:56 > > > > > > > > > > On Mon, Aug 10, 2020 at 10:41:10AM +0800, Xu Yilun wrote: > > > > > > The feature id is stored in a 12 bit field in DFH. So a u16 variable is > > > > > > enough for feature id. > > > > > > > > > > > > This patch changes all feature id related places to fit u16. > > > > > > > > How much bigger does it make the kernel? > > > > > > The patch changes the definition of feature id from u64 to u16, and will > > > make the kernel slightly smaller. > > > > Unlikely. > > Most of the structures will gain a 'pad' field. > > Using u16 for function parameters and results almost certainly > > requires instructions to mask the value. > > Any arithmetic on u16 will require masking instructions on > > (probably) all architectures except x86. > > > > Using 'unsigned int' is probably best. > > > > u16 is never a good idea unless you are defining enough > > of them in a structure (eg as an array) to reduce the > > structure size below some threshold. > > (Or are matching some hardware layout.) > > I got it. Thanks for your detailed explanation. I think we may change them to > u32. Is it the same case for u8? Think we may also change the dfl_device_id.type. Loosely 'yes' but it isn't worth the churn of 'random' changes. And they aren't often passed to/from functions - which I'm 98% sure requires masking. I commented because the compiler was going to add pad fields after your u16 values - so you'd get do space saving and probably more code. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
