# Re: \[PATCH v10 2/3\] tty: hvc: pass DMA capable memory to put\_chars()

- From: Xianting Tian \<hidden\>
- Date: 2021-10-14 08:35:06 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2021/10/1541605
- Thread: https://ratatoskr.run/linuxppc-dev/2021/10/1536312/t.md

```text

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 */

    /* --- cacheline 9 boundary (576 bytes) --- */
    char                       outbuf[0];            /*   576 0 */

    /* size: 576, cachelines: 9, members: 17 */
    /* sum members: 476, holes: 2, sum holes: 100 */
};


>
> thanks,
>
> greg k-h
```