Re: [PATCH v10 2/3] tty: hvc: pass DMA capable memory to put_chars()

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Oct 14, 2021 at 04:34:59PM +0800, Xianting Tian wrote:
> 
> 在 2021/10/10 下午1:33, Greg KH 写道:
> > On Sat, Oct 09, 2021 at 11:45:23PM +0800, Xianting Tian wrote:
> > > 在 2021/10/9 下午7:58, Greg KH 写道:
> > > > Did you look at the placement using pahole as to how this structure now
> > > > looks?
> > > thanks for all your commnts. for this one, do you mean I need to remove the
> > > blank line?  thanks
> > > 
> > No, I mean to use the tool 'pahole' to see the structure layout that you
> > just created and determine if it really is the best way to add these new
> > fields, especially as you are adding huge buffers with odd alignment.
> 
> thanks,
> 
> Based on your comments, I removed 'char outchar',  remian the position of
> 'int outbuf_size' unchanged to keep outbuf_size and lock in the same cache
> line.  Now hvc_struct change as below,
> 
>  struct hvc_struct {
>         struct tty_port port;
>         spinlock_t lock;
>         int index;
>         int do_wakeup;
> -       char *outbuf;
>         int outbuf_size;
>         int n_outbuf;
>         uint32_t vtermno;
> @@ -48,6 +57,16 @@ struct hvc_struct {
>         struct work_struct tty_resize;
>         struct list_head next;
>         unsigned long flags;
> +
> +       /*
> +        * the buf is used in hvc console api for putting chars,
> +        * and also used in hvc_poll_put_char() for putting single char.
> +        */
> +       char cons_outbuf[N_OUTBUF] __ALIGNED__;
> +       spinlock_t cons_outbuf_lock;
> +
> +       /* the buf is used for putting chars to tty */
> +       char outbuf[] __ALIGNED__;
>  };
> 
> pahole for above hvc_struct as below,  is it ok for you?  do we need to pack
> the hole? thanks
> 
> struct hvc_struct {
>     struct tty_port            port;                 /*     0 352 */
>     /* --- cacheline 5 boundary (320 bytes) was 32 bytes ago --- */
>     spinlock_t                 lock;                 /*   352 4 */
>     int                        index;                /*   356 4 */
>     int                        do_wakeup;            /*   360 4 */
>     int                        outbuf_size;          /*   364 4 */
>     int                        n_outbuf;             /*   368 4 */
>     uint32_t                   vtermno;              /*   372 4 */
>     const struct hv_ops  *     ops;                  /*   376 8 */
>     /* --- cacheline 6 boundary (384 bytes) --- */
>     int                        irq_requested;        /*   384 4 */
>     int                        data;                 /*   388 4 */
>     struct winsize             ws;                   /*   392 8 */
>     struct work_struct         tty_resize;           /*   400 32 */
>     struct list_head           next;                 /*   432 16 */
>     /* --- cacheline 7 boundary (448 bytes) --- */
>     long unsigned int          flags;                /*   448 8 */
> 
>     /* XXX 56 bytes hole, try to pack */
> 
>     /* --- cacheline 8 boundary (512 bytes) --- */
>     char                       cons_outbuf[16];      /*   512 16 */
>     spinlock_t                 cons_outbuf_lock;     /*   528 4 */
> 
>     /* XXX 44 bytes hole, try to pack */

Why not move the spinlock up above the cons_outbuf?  Will that not be a
bit better?

Anyway, this is all fine, and much better than before, thanks.

greg k-h